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Testing key underlying assumptions of respondent driven sampling within a real-world network of people who inject drugs

By Ryan Buchanan, Charlotte Cook, Julie Parkes, & Salim I Khakoo

The World Health Organization has recently set a target for the global elimination of Hepatitis C. However, to monitor progress it is necessary to have accurate methods to track the changing prevalence of Hepatitis C in populations that are most affected by the virus. People who inject drugs are a marginalized and often hidden population with a high prevalence of Hepatitis C. As such, tracking Hepatitis C infections in these populations can be difficult. One method to do just this Respondent Driven Sampling or RDS. However, prevalence estimates made using RDS make several assumptions and it is difficult to test whether these assumptions have been met.

However, our recently published article in the International Journal of Social Research Methodology describes a novel way to do just this. This blog shares some of the challenges faced in doing this work and how, by using novel social network data collection techniques, we were able to test some of the assumptions of RDS in an isolated population who inject drugs on the Isle of Wight in the United Kingdom. However, before delving into how we did this, a brief introduction to the RDS method is necessary.

RDS requires that researchers start with a carefully selected sample within a target population. These individuals (called seeds) are asked to refer two or three friends or acquaintances to researchers who are also eligible to take part. These new participants are asked to do the same and recruitment continues in this way through ‘waves’ until the desired sample size is achieved. Then, using appropriate software, the data collected during the survey about each persons’ social network allows for the estimation population prevalence (e.g., how common Hepatitis C is in the population of people who inject drugs).

Using RDS to estimate the prevalence of Hepatitis C among the population of the Isle of Wight, we hypothesized that the treatment program was closer to achieving the elimination of the virus than the available data suggested.

However, concerns remained about the potential flaws of RDS and we were interested in how one could develop methods to assess these flaws. Here our study on the Isle of Wight presented a unique opportunity. The small island population made it possible to map the social networks connecting people who inject drugs through which the sampling process passes. With this network ‘map’ it would then be possible to test whether some of the assumptions underlying the method had been met.

To achieve a mixed methods social network study was run alongside the main survey. Interviews were conducted with people who inject drugs on the Island as well as the service providers who worked with them. These interviews explored how they were all interconnected. Survey participants were also asked about their social networks which then aided in the construct of a representation network through which the ‘waves’ of the sampling process passed.

Unsurprisingly, many survey participants were unenthusiastic about identifying friends and acquaintances who also inject drugs. Instead, unique codes for each individual described were utilized. These comprised of their initials, age, hair colour, gender and village or town where they lived. Participants were asked about each individual they described e.g., how frequently do they inject or if they use needle exchange services? In this way a picture of the social network of people who inject drugs on the Isle of Wight was gradually built up which provided insights into this population even though some of the target population hadn’t come forward to directly participate in the survey.

With this ‘map’ in-hand and the personal information collected we collected it was possible to test some of the assumptions of RDS like (1) if the target population for the survey are all socially interconnected or (2) if members of the population are equally likely to participate in the survey.

Read the full article is the IJSRM here.

The researchers would like to thank the individuals who came forward and took part in this study, the community pharmacists who provided research venues in towns and villages across the Island, and the study funders (NIHR CLAHRC Wessex).

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