Attitudes towards transactional data donation and linkage in a longitudinal population study: evidence from the Avon Longitudinal Study of Parents and Children

Background: Commercial transaction records, such as data collected through banking and retail loyalty cards, present a novel opportunity for longitudinal population studies to capture data on participants’ real-world behaviours and interactions. However, little is known about participant attitudes towards donating transactional records for this purpose. This study aimed to: (i) explore the attitudes of longitudinal population study participants towards sharing their transactional records for health research and data linkage; and (ii) explore the safeguards that researchers should consider implementing when looking to request transactional data from participants for data linkage studies. Methods: Participants in the Avon Longitudinal Study of Parents and Children were invited to a series of three focus groups with semi-structured discussions designed to elicit opinions. Through asking participants to attend three focus groups we aimed to facilitate more in-depth discussions around the potentially complex topic of data donation and linkage. Thematic analysis was used to sort data into overarching themes addressing the research questions. Results: Participants (n= 20) expressed a variety of attitudes towards data linkage, which were associated with safeguards to address concerns. This data was sorted into three themes: understanding, trust, and control. We discuss the importance of explaining the purpose of data linkage, consent options, who the data is linked with and sensitivities associated with different parts of transactional data. We describe options for providing further information and controls that participants consider should be available when studies request access to transactional records. Conclusions: This study provides initial evidence on the attitudes and concerns of participants of a longitudinal cohort study towards transactional record linkage. The findings suggest a number of safeguards which researchers should consider when looking to recruit participants for similar studies, such as the importance of ensuring participants have access to appropriate information, control over their data, and trust in the organisation.


Introduction
Data linkage, or the linking of two or more different sources of information about the same phenomena of interest 1 , is an efficient and cost-effective method for carrying out epidemiological research 2 . It is particularly beneficial for longitudinal cohort studies, an observational research method where data about the same participants are gathered repeatedly over a period of years or even decades 3 , as it allows for the study of links between a vast range of behaviours, medical conditions, environmental factors, genes, lifestyle choices and health outcomes 4 . Whilst cohort studies use advanced data collection protocols, a large amount of information on the daily behaviours and lifestyles of participants is collected by self-report and hence subject to missingness and/or bias 5 . Objectively recorded routine records provide a means to quantify and address these concerns. For this reason, funders of UK longitudinal population studies (LPS) have identified record linkage as a strategic priority, and within this are encouraging studies to investigate linkages to a wide range of novel sources 6-8 .
Commercially collected transactional records, such as banking, phone, internet and retail loyalty card records, present a novel opportunity for LPS to collect objective information on participants' behaviours. However, contemporary data science approaches recognise that using potentially sensitive information such as these needs to be based on rigorous co-design frameworks involving a wide range of stakeholders: insights from this process can then be used to identify the bounds and safeguards needed to make the data use acceptable, and to use diplomacy traditions and means to help reconcile stakeholder views into an acceptable data use framework [9][10][11] .
As a first step towards developing an ethical and privacy preserving framework for linkage of commercial transactional datasets into LPS databanks, this paper uses focus groups to investigate the attitudes and understanding of participants in the Avon Longitudinal Study of Parents and Children (ALSPAC).
Public attitudes towards the donation of personal data One of the principal aims of the General Data Protection Regulation 12 is to afford greater control over personal data to the individual 13 , and the right to data portability (Article 20) has provided a legally mandated mechanism that can be used for the general public to obtain and donate their individual digital footprint data (an individual's trail of online data 14 ) for research purposes 15 . We refer to data donation as 'an act of active consent of an individual to donate their personal data for research' 16 . However, previous research has demonstrated that participants do not always have sufficient knowledge or understanding about their personal data or what they can be used for 17,18 , suggesting that not all individuals are well-informed to make decisions about donating personal data.
In light of these findings, there is a growing body of literature seeking to explore public attitudes towards donating personal data for research. This has mostly been in the medical records domain, although researchers have also recently begun to explore views on the use of itemised phone call records 19 . In their systematic reviews, Aitken et al. 17 and Stockdale et al. 18 found that, despite a common lack of knowledge and awareness about the value of patient data for research, there is a general willingness to share medical data. This was linked with a sense of obligation, altruism and expectation that the data used for research will contribute towards knowledge for the greater public good.   19 found that only 3% of participants in their study were aware that mobile phone data was being used in health research. However, 62% supported the use of their data for this purpose. Similarly, Skatova & Goulding 16 demonstrated that individuals are willing to donate their loyalty card data to research benefiting the public good. The decision to donate personal data was associated with three distinct reasons: being a good member of society, prosocial motivation and understanding the reasons for donating personal data.
However, others [20][21][22][23] have demonstrated that there is a sense of fear towards data donation amongst the general public, connected with concerns around hacking, identify theft and the misuse of patient data for financial gain, as well as the need to both protect individual privacy and to trust the entity with which the data is shared. Whether individuals support sharing their medical records is also dependent on factors such as confidentiality, presence of safeguards to prevent the misuse of data, perceived control over how data is used, and the opportunity to provide explicit consent for sharing data.

Public attitudes towards data linkage
There is considerable literature around consent for data linkage for research purposes 18,24 . From a researcher's perspective, obtaining consent for linkage of records is likely to require participant identification, which can be viewed as a breach of confidentiality, and non-response to consent requests could introduce sample bias 24 . Furthermore, a longstanding counterargument against the need for consent is that the risk of nonresponse could hinder scientific progress and 'undermine the public good' 25 . These arguments have often been used to reject the typical opt-in consent process normally used by researchers when linking previously collected data into longitudinal population studies 2 , in favour of opt-out consent.

Amendments from Version 1
In the revised version, we explained why we are focusing on loyalty cards data, added description of gender and ethnicity of our participants, we have broken up the discussion into six 'findings' and associated 'recommendations' in order to make this clearer for the reader and added more information about Avon Longitudinal Study of Parents and Children. Further, we added suggested by reviewers references (e.g., Jones K, Daniels H, Heys S, Ford D: Public Views on Using Mobile Phone Call Detail Records in Health Research: Qualitative Study. JMIR mHealth and uHealth. 2019; 7 (1).).

REVISED
The opinions of the general public on consent for data linkage appear more divided. A previous qualitative interview study exploring the views of ALSPAC participants (n=55) on consent 26 , revealed that the type of data proposed for data linkage was an important consideration, with fears about the sensitivity of data, and whether the individual could be stigmatised. Stigmatisation could happen, for example, through the linkage of teenage pregnancy and state benefits data, and the linkage of mental health records and criminal records. For these reasons, opt-in consent was preferable amongst some participants 26 . Similar views were found by Davidson et al. 21 in a qualitative study with workshops exploring public attitudes (n=73) towards the acceptability of cross-sectoral data linkage, such as health, social care and education data, where for instance, participants feared discriminatory treatment by agencies for having a criminal record.
The degree to which the topic or outcome of the research was considered beneficial for the public good was also influential on participant views in both studies 21,26 . For example, the linkage of birth weight and future health outcomes was considered by some to not require opt-in consent due to the potential benefits 26 . Participants (n=26) in a qualitative interview study by Xafis 27 were also more likely to express that consent was not required when they were aware of the public benefits of proposed research projects. Furthermore, participants in this study stated that consent for data linkage was not required when they held trust in the data linkage organisation, whereas the predominant view was that consent should be sought when researchers carry out the data linkage process and have access to identifying information.
Other concerns included whether linked data could be sold for commercial or political purposes, and the increased likelihood of hacking and data misuse due to the way in which more people would have access to the data 21 . However, fears were linked with a lack of awareness around data de-identification, and when participants were assured that identifying information would not be revealed, many were less nervous about data linkage 21 . Likewise, confirmation about the anonymity of data influenced participant decisions about consent in the study by Audrey et al. 26 , with some believing it was not necessary if data was analysed at population level.

Aims of the study
Despite the growing research interest in public opinion on linkage of health-related data, public attitudes towards sharing transactional records for data linkage remain an unexplored area, specifically in the context of longitudinal population studies and health research. As a first step in co-designing a conceptual framework for longitudinal population studies, we invited ALSPAC participants to focus groups to collect their opinions on linking transactional records for research, specifically retail loyalty cards and banking cards data. These two data types were emphasised as ALSPAC are considering mechanisms for using these data to enable new research possibilities. We studied participant attitudes towards ALSPAC requesting access to transactional records to link with their individual data in the ALSPAC databank for use in future research. This paper presents the results of these conversations in response to the following overarching research questions: 1. What are the attitudes and concerns of ALSPAC participants towards providing consent for accessing their personal retail and banking records for linkage of these data into the ALSPAC databank? 2.
What are the safeguards that should be put in place by researchers to address any concerns raised by participants?

ALSPAC
ALSPAC is a multigenerational prospective birth cohort study. ALSPAC recruited pregnant women resident in and around the City of Bristol (South-West UK) and due to deliver between 1st April 1991 and 31st December 1992. There were an initial 14,541 enrolled pregnancies comprising 14,676 foetuses (for these at least one questionnaire has been returned or a "Children in Focus" clinic had been attended by 19/07/99). These pregnancies resulted in 14,062 live births and 13,988 children alive at 1 year. From age 7, attempts were made to recruit additional cases who were eligible under the original sample definition 28,29 ). By age 24, an additional 913 index children had enrolled. The total sample size for analyses using any data collected after the age of seven is therefore 15,454 pregnancies, resulting in 15,589 foetuses. Of these, 14,901 were alive at 1 year of age 30 . The cohort has been followed intensively from birth through self-completed questionnaires and attending clinical assessment visits. ALSPAC has built a rich resource of phenotypic and genetic information relating to multiple genetic, epigenetic, biological, psychological, social, and other environmental exposures and outcomes. ALSPAC is a globally accessible research resource, which also allows recall studies: including those considering participant understanding, expectations and acceptability of different research designs. The ALSPAC resource has an online data dictionary (http://bristol.ac.uk/alspac/researchers/our-data/) and a public access mechanism (http://bristol.ac.uk/alspac/researchers/ access/)."

Study design
We used focus groups as a method of data collection, incorporating semi-structured discussions to elicit participant attitudes towards using transactional data for public health research, and the linking of transactional records into longitudinal population studies. The focus groups were run in three parts, each a month apart. This time gap was created to help participants digest information about personal data as well as any issues that arose in the discussion. We expected that with this time gap, participants would come to informed and deliberate opinions as to whether it is appropriate to share specific types of their personal data with ALSPAC for academic research, and also comment on more or less appropriate routes for research. Each part of the focus group had a different discussion topic. Each participant was invited to all three parts of focus groups, albeit not all participants took part in each part because of individual reasons unrelated to the study. Written materials for each part were not provided to participants in advance.
For Focus Group Part 1 and Focus Group Part 3, two separate focus groups were conducted. For Focus Group Part 2, only one focus group was held, due to difficulties in recruiting a sufficient number of participants. Each focus group lasted between 60 and 120 minutes. Authors AS and AB conducted the focus groups with a member of the ALSPAC family participation team also present. AS is a research fellow external to ALSPAC. AB is the ALSPAC data manager but had not previously met any of the participants in this study. Focus groups were audio-recorded using dictaphones and AS took field notes as well as photographs from focus group one showing how participants had sorted various categories of data.
In Focus Group Part 1, we explored dimensions that underpin barriers for data linkage (e.g., trust in how the data will be handled, understanding what will happen with the data etc), as well as attitudes to sharing and using personal data in general, both for research and commercially. This focus group introduced participants to different types of personal data as well as issues around sharing personal data in general. Since previous research showed that individuals know little about their personal data 17,18 , participants were briefed on the most common types of data that can be collected about them through digital means and asked to group or order according to their own choice of categories. For instance, from least sensitive to most sensitive. Types of data presented to participants on cards included: mobile phone; car GPS; electricity use; physical activity (exercise); browsing history; search history; click history; car speed records; cycling camera video; sleep patterns; bank transactions; online shopping history; age, gender, marital status; medical records; online dating history; social media; loyalty card data; mobile phone use; broadband use; and home address. This was followed by a more specific discussion about sharing financial and retail loyalty cards records with ALSPAC for academic research. Participants alternated between working in pairs and group discussion to facilitate interaction.
In Focus Group Part 2, we explored attitudes to sharing personal data that were discussed by participants in more detail using an interactive game approach. Participants were firstly presented with a set of 'info cards', which explained the different elements of the record linkage process, such as 'data protection', 'informed consent', and 'data reuse'. They were also given a set of 'issue cards' with various beliefs associated with record linkage such as 'data anonymisation is a myth' and 'individuals want to be aware of how their data is being used'. For both sets, participants were asked to pick one or two cards, which they found interesting or controversial and explain to the group. They were then provided with 'story cards' which presented the viewpoints of various fictional individuals involved in or potentially affected by record linkage. For instance, an ALSPAC staff member, a policy maker and a person with a rare disease. Participants were asked to discuss whether they identified with a particular story card, or whether they found a story card controversial and present this to the group. Finally, we asked participants to make decisions on whether to grant permission to specific (hypothetical) research projects that would use data linkage and various forms of consent.
In Focus Group Part 3, we presented participants with a conceptual framework of how the linkage can be done within ALSPAC. The framework included different options that were discussed with participants: e.g. the data can be shared anonymously with authorised third-party (non-ALSPAC) academic researchers; the past retail or banking records can be linked but not the future data. We focused on the acceptability of linkage scenarios and the different ways in which the data can be shared. In particular we were interested in exploring the attitudes towards retrospective versus prospective data collection given ethical views suggesting that asking consent for retrospective data is more acceptable than asking consent for ongoing prospective harvesting of information. Full protocol of the focus groups is available on request from the first author. The focus groups were conducted in April -June 2018.

Ethical approval
Ethical approval for the study was obtained from the ALSPAC Ethics and Law Committee and the Local Research Ethics Committees. Informed consent for the use of data collected in this study was obtained from participants following the recommendations of the ALSPAC Ethics and Law Committee at the time (a University of Bristol faculty ethics committee).

Participants, recruitment strategy and compensation
We used convenience sampling for recruitment: a sub-set of ALSPAC participants (n=600) from the index generation were invited to participate when aged 25-26 years old. The sub-set was randomly selected from a pool of individuals with a Bristol postcode (in order to facilitate attendance at multiple workshops) and who had a valid email address on file. Standard filters were also applied, with people who had died, withdrawn, or asked for a break from participation not included in the pool.
We sent an email invitation letter and an information sheet about the nature of the study. Any invited ALSPAC index participant could take part, whether they used services like banks, held loyalty cards, or not. All contact with participants was managed by ALSPAC administrative staff, who recruited the participants and arranged suitable dates for the focus groups. Although focus groups were designed so that the same participants attended all three, not all participants from Focus Group Part 1 were able to attend subsequent groups. In this case, we invited new participants who expressed a desire to participate. A total of 20 participants attended the focus groups (male=11; female=9). All participants were of white British ethnicity. Five participants attended all three focus groups; eight attended two focus groups; and seven attended one group. In the first Focus Group Part 1, ten participants attended (male=5; female=5). In the second Focus Group Part 1, five participants attended (male=3; female =2). In Focus Group Part 2, nine participants attended (male=4; female=5). In the first Focus Group Part 3, eight participants attended (male=5; female=3). And, in the second Focus Group Part 3, three participants attended (male=1; female=2).
All participants were asked to provide informed consent before taking part and were able to withdraw at any point. Participants were also assured that their contributions would be pseudonymised during analysis and published outputs would be anonymised. As part of the informed consent process, participants were asked whether they were happy for discussions to be recorded and for recordings to be stored with ALSPAC for potential future use. Participants were not obliged to participate in all three parts of the focus groups and were reimbursed for each participation with a £10 voucher, as well as for any travel expenses incurred. Participants were rewarded with an extra £5 if they participated in all three focus groups. Focus groups took place in the ALSPAC 'focus clinic', which is a study assessment centre.

Data analysis
Recordings were transcribed by a university-authorised transcription service. Inductive thematic analysis was initially carried out by author KS, which allowed themes to be linked with the data itself, rather than with a pre-existing coding framework 31 . After becoming familiar with the data, initial codes relating to participants' concerns and associated safeguards were generated and merged where appropriate.

Results
This study is a first step towards understanding participants' attitudes to linking transactional records into longitudinal population studies for public health research. We aimed to investigate (i) whether participants understand why linking loyalty cards and banking data into the study databank is useful for public health research, and if they do, what are the best and most efficient ways of explaining the utility of public health research with transactional data. We further explored (ii) different reactions that respondents might have towards transactional records linkages and the spectrum of individual reactions. Finally, we were interested whether participants are prepared to agree to such data linkage and if so, (iii) given participants' concerns, what safeguards need to be put in place to make transactional data linkage possible.

Understanding
We first discuss whether participants understood why there is a need to link their personal data into a longitudinal population study databank, and the most common concerns associated with data linkage. In general, there was a range of opinions on whether sharing personal transactional data was acceptable. Some participants were indifferent about sharing certain types of transactions: "I really don't mind any of the stuff that's like clothing store, petrol station, food." [FG2] Others raised various concerns about data linkage and data sharing, which we discuss below.
The acceptability of the level of data sharing often appeared to be contingent on the context and purpose of data sharing, which showed that it is important for participants to be informed about the details of data being collected and who the data is shared with. Even though sharing data for commercial purposes was never an option in our scenarios, given the general public narrative around personal data and that this is not permitted within ALSPACs participant framework 1 , sharing data for private benefit was still discussed at the focus groups. In particular, there appeared to be a misunderstanding with regards to the potential of sharing transactional data with commercial companies or for profit. For example, a common concern was whether credit history, loans or insurance may be affected if companies could access transactional data: "Could there be a chance that that might impact the deals you get from your bank maybe?" [FG1] "Will it affect things like your credit history and stuff?" Individuals were more prepared to share data if they felt it would be beneficial for society, rather than if it is for political or commercial gain, and the general consensus was that donating data to researchers is purposeful: Trust An important factor in the decision whether to share transactional data was who the data is shared with. There was a distinct difference in attitudes to sharing personal data with ALSPAC, whom participants trusted, vs external researchers. Several participants alluded to the high levels of trust they placed in the ALSPAC (aka Children of the 90's) researchers, having been part of the cohort since birth and having experienced high standards of research practice, and they felt happy to provide them with their transactional data: "That's one of the things I really like about Children of the 90's, they collect all of this data but they keep it anonymous and confidential, always." [FG1] This was also linked in part to the knowledge of how their identity was protected through pseudonymisation processesreferred to as being registered as an ID number: "You're just a number and you know your shopping habits or your banking habits are just part of a bigger data search. That feels, that feels safer doesn't it?" [FG2] One participant also highlighted that they were more trusting of the motivations of ALSPAC researchers: " However, there were general concerns around whether external researchers could be trusted not to misuse data. This fear may have been stoked by an increasing awareness amongst the general public about the power and risks of mis-use of personal data, with participants alluding to Cambridge Analytica scandal as an example of how personal data could be harnessed for political manipulation. The focus groups were conducted only a couple months after this high profile case of social media data misuse was revealed: In order to trust third-party data users, participants stated that they would like to be able to access information on who they are sharing information with and how trustworthy the organisation is: One participant suggested that they could be presented with a range of categories of research that they could opt in or out of: "I think if you opted in and it was under the umbrella of physical and mental health research and innovation." Participants highlighted that it is important to explain what 'transactional data' means before asking for consent to share the data as individuals felt that being asked to consent in general would cause potential participants to decline involvement in the study as they would prefer to have control over which types of data are shared: "If you just say transactional data, if someone doesn't really want online stuff within that they will just say no to the whole thing. Whereas they might have been happy for the loyalty cards stuff." [FG2].
However, they were aware that extracting the data themselves and passing this onto researchers every time could be potentially time-consuming and prevent them from sharing data. Participants suggested one solution for how consent for sharing various types of data could be obtained efficiently, with various categories of data and an accompanying explanation, which they could select if they were happy to share. For instance, there were a number of sensitive categories of data that participants said they were unhappy about sharing, due to fears about what data could reveal about them, such as information geospatial information that can be derived from shopping data:

"I don't really think I want people to know where I shop and how often just because, I don't know, it's a bit personal." [FG3]
Participants also indicated that they considered some types of information to be more private and which they would be reluctant to share. For instance, in Focus Group 1a and 3, two participants indicated that data revealing salary was more private; two participants in focus group 3 believed that some members of the public would be worried whether their data could reveal information about gambling; and one participant in focus group 3 was concerned about sharing transactions related to purchased medicines:

"I don't really mind if people know I'm buying shoes or meat and groceries and stuff. But I think the only sensitive, I don't know whether it should be sensitive or a privacy issue, is prescription medicine, contraception maybe." [FG3]
Individuals feared that the more granular the data is, the less likely data would remain anonymous: Following discussions on consent for categories, the consensus was that researchers should then extract these types of data according to participants' wishes:

"I feel like I personally would rather like have a big list of, I don't know, 'can we access this?' loyalty card, tick yes. And then you do it." [FG3]
During the focus groups, we gathered participants' views on whether requesting permission to access retrospective banking or loyalty card data at any point in time is more acceptable versus requesting permission to access prospective data about an individual's transactions. Participants raised concerns associated with both future and retrospective data collection, however consenting to share retrospective data collection was seemingly preferable amongst most individuals: In general, the notion of 'live' data sharing was not comfortable for participants and they wanted information about the concept of 'live' data collection and reassurance that they cannot be tracked: As regards to forms of consent, participants were largely happy with an initial opt-in consent to extract the records and include them into the ALSPAC resource, but with an ongoing option to opt-out for the subsequent reuse of data: There was, however, a feeling that this was a futile process linked with a sense of resignation and acceptance from some that data always leaks out and that no one can do anything about this, as this is part of life today. Thus, participants may require reassurance and details relating to data security measures:

"I don't think researching who's taking your data really matters anymore because you have hacks in data and breaches these days." [FG2]
Specific reassurances that participants would like included consent, encrypted and anonymous data, and that any identifying information will remain with the ALSPAC team: 3 Personal transactional data can be linked with data previously collected by Children of the 90s and used for academic research with opt-in consent. These records can be re-used for different C90s projects, and participants have the right to object (opt-out) and stop this from happening.

Discussion
The results of these focus groups represent contributions to the development of 'ethical parameters', a process which Metcalf and Crawford 32 suggest is crucial as data science methodologies develop. Below we discuss the main findings and associated recommendations.
Finding 1: A lack of awareness as to why transactional data is valuable for health research Perhaps unsurprising given the novelty of the topic, there was little to no awareness amongst participants of the value of transactional data for data linkage with their ALSPAC records. This is comparable to findings from a large-scale qualitative study, which found that the public have low levels of understanding about the uses of their patient data for health research 33 . Attitudes amongst the focus group participants evolved as the proposed usage and benefits that transactional data could bring to ALSPAC research became clearer. Initially, many expressed surprise at the concept and an unfamiliarity as to how this could inform research. Those participants who were cautious about data sharing initially transitioned to a willingness to donate this type of data: a change linked with a desire to help find cures for diseases, or benefit society in more general terms coupled with the clarification of the data processing activities and the safeguards that could be deployed. These findings reflect those of Skatova and Goulding 16 , who found that willingness to engage in data donation was linked with an understanding of the purpose of the research and the prosocial motive of an individual.

Recommendation 1
Prior to approaching participants for consent, researchers should consider the need to emphasise the value of transactional data and the potential impact of the proposed research. It is unlikely however, from this evidence based on a small and typically committed participant group, that all ALSPAC participants would accept this use of their data. To reassure those individuals, fair processing information materials would need to also emphasise why this new activity was in keeping with the broader and 'traditional' remit of the study; and then to make clear this was optional activity based on explicit consent for those accepting of this use of their data (see Finding 2 below). This otherwise raises concerns that the activity could be perceived as a shift of study direction, which may threaten wider participation and trust.
In turn, this may reinforce the value in approaches where studies (such as ALSPAC) emphasise that 'enrolment' does not commit any participant to undertaking any assessment or providing data (i.e. taking part in any assessment is optional, providing an item of data within any assessment is optional). This may encourage a feeling of choice and an acceptance of innovation within study activities even where the activity is not personally acceptable; although this may need to be offset against a potential feeling of obligation to take part by very committed participants. These considerations remain under-explored, but reinforce the need for clear messaging on the purpose of data collection and that taking part is optional. This also highlights the benefits of studies operating parallel 'innovation' studies (e.g. the Understanding Society Innovation Panel 34 ) where innovative approaches can be tested in an accepting sample.
However, subsequent discussions showed that although increased knowledge of the uses of transactional data for linkage seemed to encourage positive reactions towards donation, this was also accompanied by a range of concerns and queries about what the process would entail and its potential repercussions, which are described below.
Finding 2: Participants need to maintain control over personal data sharing for research A common theme running throughout the focus groups and linked to a number of concerns was the need to maintain control. Bradwell and Gallagher 35 point out how individuals 'surrender control' when sharing personal information. They suggest that, in order to allow participants to regain control, there should be a move towards a more 'democratic use of personal information' with a 'bottom-up policy driven by collectively negotiated norms and rules'. This reflects previous findings in ALSPAC where participants suggested safeguards relating to the study use of routine health and government records 4 .

Recommendation 2
Participants in our study suggested a number of ways in which they could maintain control over their data sharing, linked with various consent mechanisms. Firstly, a number of participants expressed the need for control over the type of research their data is used for, and secondly, control over the types of transactional data they donate, in particular, purchases or transactions they viewed as sensitive, including those involving third-parties. Therefore, researchers should consider providing an opt-in consent list of various categories of research and categories of transactional data.
Finding 3: There are differences in attitudes to sharing different types of transactional data When discussing the types of transactions that would be visible to researchers, participants appeared to be split over whether sharing certain types of data would cause them concern and highlighted which parts of their data should not be shared.
In particular, participants put more emphasis on protecting their banking than loyalty cards data. The importance of how sensitive the information that can be revealed through shared data influenced different attitudes between sharing loyalty card data and banking records (e.g., salary information from banking records was discussed as very sensitive). Our findings are in line with previous research 16 suggesting that people are more concerned about protecting their banking records as compared to loyalty card data when sharing data in general. This concern was sometimes linked with fears that their identity could be exposed, or that behaviours could impact their credit scores.

Recommendation 3
This finding highlights the need for longitudinal population studies to explain detailed reason for linking different types of transaction data. Further, the difference between different data types is especially important when explaining the process of linking the data, and the ways in which participant identifiers are only visible to the data linkage organisation, in this case ALSPAC, and not external researchers requesting the data 27 .
Finding 4: Granularity of the data can affect decisions to share There was greater concern about data linkage amongst participants when the data appeared more granular. This finding reflects previous research on data sharing 36 suggesting that individuals are more likely to assign higher value to protect more granular, less anonymous data whilst making a decision to share personal data with third parties. Furthermore, the same study demonstrated that the general public perceives as less risky sharing personal data with universities for academic research compared to governments for planning or administrative purposes, or private companies for either research or profit-making purposes.

Recommendation 4
Despite ALSPAC's wider assurances that participant data will not be used for profit, conversations about sharing personal data do commonly raise fears by association with sharing commercial data for profit and it is important to clarify to participants that this will not take place. This also places emphasis on the need for rigorous data processing pipelines (and the clear description of these) where the transformation of granular and disclosive data to structured data with low disclosure potential is handled by study data managers operating in a trusted role.
Finding 5: There are differences in attitudes to share retrospective vs prospective data The concept of 'live' data and its association with tracking particularly worried participants, rather than continuous-in-time data sharing, which has little resemblance to 'live' tracking.
Studies will need to consider if they seek retrospective and/ or prospective data collection and explain that "live" data tracking is not required for research purposes. Although the predominant view amongst participants in this study was that they would prefer to donate their data retrospectively rather than prospectively, concerns were expressed towards both.

Recommendation 5
We suggest that consent forms should provide participants with the opportunity to choose whether to donate retrospective or future transactional data too, with information on the risks associated with both options explained. Similarly, researchers should consider providing options to consent on an opt-in basis and opt-out. Researchers should consider that the most practicable route for extracting these records will be via participants initiating 'right to portability' requests which will, by necessity, be opt-in and retrospective at the first instance, but discuss with participants whether they consent to researchers to access future data.
Finding 6: High levels of trust in a research organization are crucial to encourage data sharing for research A final common theme running through the focus groups was that of mistrust in the general contemporary use of personal information and digital footprint data. This echoes findings from a recent qualitative study interviewing 2,259 adults online 37 , where participants portrayed a picture of a society distrusting of data sharing, associated with the increasing awareness of misuse, such as data harvesting. However, participants in our study expressed high levels of trust in ALSPAC staff and were reassured to learn that ALSPAC would carry out the processing and linkage of the data;

Recommendation 6
Despite the levels of trust participants have in ASLPAC, they will require reassurance that any external researchers using their data will uphold the same standards, particularly in regard to encryption and anonymity.

Strengths and limitations
According to the authors' knowledge, this study is the first of its kind to qualitatively explore attitudes towards transactional record sharing and linkage in the context of longitudinal research. The findings therefore are novel and provide an initial step towards the development of an evidence-based conceptual framework guiding researchers looking to recruit participants into transactional record linkage studies. The design of the study involving three focus groups for the same participants allowed time for reflection and to form opinions on this novel field.
Limitations of the study include the small sample size, that all participants were approximately the same age (reflecting the ALSPAC index sample) and the lack of consistent participation of the same individuals across the three focus groups, which meant that more in-depth discussion around the topic may have been limited. The nature of focus groups may have also meant that the expression of participants' true attitudes was limited by social desirability bias. As participants have been part of the ALSPAC cohort since birth, they were likely to be more informed of the benefits and consequences of sharing their data for research, as well as certain processes, such as de-identification, than the general population, and thus likely to suggest known safeguards. The reported attitudes are likely to be shaped by their continuing trust and acceptance of involvement in longitudinal research; this was not a representative group of all study participants, and the value of these insights lies more in terms of helping inform process design and communication strategies rather than as an indication of how many participants would accept this use of their data. Therefore, the results are mostly relevant to longitudinal population studies participants rather than the wider general public. Finally, it is plausible that because we discussed different types of data with participants, and specifically different types of transactional data, the results are biased by the order in which different data types were discussed. For example, participants might be more likely to express positive attitudes about data linkage in general if and when loyalty cards data were presented to them before banking data, and vice versa. Due to sample size we do not have sufficient evidence to provide conclusive evidence on this matter.

Future research
Future research should seek to explore the views of a more diverse range of participants from the general population towards the donation of transactional records for public health research to provide a more generalisable picture of attitudes. Follow-up interviews could also complement focus group discussions by providing an opportunity for researchers to discuss any specific issues arising from focus groups in more depth and explore any variations in opinions following a period of reflection. The use of individual qualitative interviews could enable participants to express their insights in more detail, whilst reducing the possibility of bias introduced by group consultations. The contrasts between participants' views on loyalty card data and banking data could also be a topic for future research. In particular, it would be of interest to investigate whether opinions on the use of one type transactional of data (i.e. loyalty card data) could affect an individual's views on the other type of transactional data (i.e. banking data). This could also be explored in a quantitative survey, where the type of data participants are asked about first is randomised.

Conclusions
This study provides initial evidence on the attitudes and concerns of participants currently involved in a longitudinal cohort study towards providing their loyalty card and banking transactional records into the study databank. The findings suggest a number of safeguards which researchers should consider when looking to recruit participants for similar studies. Across the three waves of workshops, participants went on a 'journey' of first seeking to understand the purpose behind the linkage of their transactional records with their previously collected ALSPAC data, and the purpose of ALSPAC research; then discussing their concerns; and finally, suggesting safeguards needed to make this form of data linkage acceptable. In particular, researchers seeking to recruit participants into transactional data linkage studies should consider the importance of ensuring participants have access to appropriate information on data usage, control over their data, and trust in the organisation. 3. Please submit your research proposal for consideration by the ALSPAC Executive Committee. You will receive a response within 10 working days to advise you whether your proposal has been approved.

Data availability
If you have any questions about accessing data, please email alspac-data@bristol.ac.uk.
The ALSPAC data management plan describes in detail the policy regarding data sharing, which is through a system of managed open access.
The study website also contains details of all the data that is available through a fully searchable data dictionary: http://www. bristol.ac.uk/alspac/researchers/data-access/data-dictionary/.
Author contributions This paper was conceived by author AS. The investigation (focus group design and methods) was developed by AS and AB. The formal analysis was conducted by AS and KS. KS and AS wrote the original draft and AB reviewed and edited the final manuscript.

Open Peer Review
in question were of a particular view?
-We have now quantified 'a number of' on page 13 and removed phrases like 'many' in other parts of the manuscript.
5. While not looking to plug my own work, I have carried out public engagement on the use of mobile phone data and the development of an ethically founded framework for the use of such data for health research. This might be of interest to the authors since they are focusing on different types of transactional data 1,2 .
-Thank you for directing us to this research. We have now referenced this in our introduction on page 4.
6. The authors might like to break up the Discussion into sub-headings, such as Main findings; What this study adds; and Recommendations.
-We have broken up the discussion into six 'findings' and associated 'recommendations' in order to make this clearer for the reader.
kind of data is low and the research participants confirmed this with their queries about how such data would be useful in health research. The overarching themes identified relate to participants' Understanding, Trust, and Control over uses of transactional data via standard consent mechanisms. This cohort in effect formed a 'community' as a result of their long-term involvement with ALSPAC and expressed great trust towards these researchers and ALSPAC. This seems to have greatly influenced their willingness to share transactional data with ALSPAC but broader reservations were evident.
Some suggestions which may improve an already excellent paper include the following: Readers would benefit from more detail about ALSPAC, particularly the fact that external researchers can request access to data held in the ALSPAC data repository for research purposes. This was not immediately clear and required the reader to make assumptions where there were references to 'external researchers'.

1.
It was not clear if written materials were made available to participants, particularly as they were intended to attend all focus group discussions. I have assumed that this was not the case.

2.
P6. "…in order to minimise authors' interpretation of the data" should perhaps be 'misinterpretation', as qualitative research does require the researcher to interpret the data.

3.
P7. The last section in the first column under 'Trust' could be moved to the section titled 'Control'.

4.
Readers would benefit from some detail around the envisaged process of initial donation of transactional data. A brief description of this process would add greater clarity and would add valuable contextual information pertinent to the issues raised and discussed.

5.
One thing that is obvious, but not mentioned in the Discussion, is the age of this cohort, which could perhaps also have some bearing on the views expressed. This is something that should perhaps be explored in future research relating to the use of transactional data.

6.
Overall, despite the great trust that this cohort quite evidently has in ALSPAC and its researchers, there seem to be more concerns about the sharing of transactional data than a willingness to share such data for beneficial health research. I was wondering if you agree with this and, if so, whether a broad statement of this kind would be useful in the Discussion.

Is the work clearly and accurately presented and does it cite the current literature? Yes
Is the study design appropriate and is the work technically sound? Yes

Are sufficient details of methods and analysis provided to allow replication by others? Yes
If applicable, is the statistical analysis and its interpretation appropriate?
-We agree that this section has elements that span sections. However, we consider that this quote would be better placed under 'trust' as in this theme we discuss participant concerns about who the data is shared with (specifically ALSPAC vs external researchers), and this specific quote refers to one participant's fears around what could happen when one set of data is linked with another set of data, and who will be able to see this data. Under the theme 'control', we focus more on the safeguards suggested by participants.
5.Readers would benefit from some detail around the envisaged process of initial donation of transactional data. A brief description of this process would add greater clarity and would add valuable contextual information pertinent to the issues raised and discussed.
-We feel that this is out of scope for this paper and this research: which was designed to identify factors to incorporate into the donation and processing of the data.
6.One thing that is obvious, but not mentioned in the Discussion, is the age of this cohort, which could perhaps also have some bearing on the views expressed. This is something that should perhaps be explored in future research relating to the use of transactional data.
-We fully agree. At the end of the strengths and limitations section on page 18-19, we have now raised this issue, and we mention the importance of including a more diverse range of participants in order to ensure generalisability in the future research section.
7.Overall, despite the great trust that this cohort quite evidently has in ALSPAC and its researchers, there seem to be more concerns about the sharing of transactional data than a willingness to share such data for beneficial health research. I was wondering if you agree with this and, if so, whether a broad statement of this kind would be useful in the Discussion.
-This is an interesting observation, and while we agree that many concerns and pragmatic issues were raised, the data suggests that participants were positive about sharing transactional data for health research where appropriate controls are deployed. The discussion was naturally focused around concerns, not benefits, as those were focus of the study. Finally, we found that at the beginning of the focus groups, participants were reluctant to share transactional data, which was linked with a limited understanding of how this novel form of data could be useful for health research. However, once they understood how this data could benefit the public good, participants moved to being in favour of sharing it and the rationale for asking: although this would not suggest all would consent to this use of their data. We address this at the start of the discussion.