Respondent driven sampling of wheelchair users: A lack of traction?

Introduction

Robust epidemiological research generally requires data collection from representative samples of the population of interest, and effective modes of sampling contact are essential¹. Such effective modes can be difficult in hard-to-reach populations where no (or inadequate) sampling frames exist. Traditional chain-referral sampling approaches are inherently biased in their participant selection methods; a bias that is compounded as recruitment waves continue. Respondent Driven Sampling (RDS) was developed to counter these biases, employing specific data collection and statistical analysis methods which enable valid population-based estimates^2–4. Despite wider adoption of RDS, and its successful application in many topic areas, methodological concerns have been raised. RDS estimates are, at times, more variable than expected⁵, and some sampling patterns appear to violate core RDS assumptions^6,7.

The prevalence of wheelchair users has rapidly increased over the last half century due in part to advancing medical care, ageing populations, increasing community supports, increased prescription of wheelchairs, and changes in attitudes to disablement such that people may feel less stigmatised about using a wheelchair^8,9. Despite this, robust epidemiological research in this group is scant¹⁰. Contacting wheelchair users in the community is challenging. Recruitment approaches are often limited to using disability organisations and personal contacts, which can differentially exclude many wheelchair users¹¹. Consequently, wheelchair users frequently constitute a ‘hidden population’, under-researched and excluded from population estimates¹². Furthermore, many countries, including New Zealand, have yet to establish registries of wheelchair users which could provide a sampling frame¹³.

Here we report our experience of applying a RDS methodology to a survey of wheelchair users in New Zealand. To our knowledge this is the first time RDS has been applied to people who use wheelchairs, and could potentially offer a significant new sampling approach in epidemiology and disability fields.

Methods

Open from October 2015 until January 2016, this study employed a short internet-based national survey. Administered through the Surveymonkey™ website, an information sheet and video were embedded within the survey preamble (see Supplementary material). The information sheet stated that informed consent was implied through the voluntary participation in the survey. Ethical approval was obtained from the University of Canterbury Human Ethics Committee (reference HEC 2015/117). Eligibility criteria included: wheelchair use as the primary form of mobility; being a New Zealand resident; aged 16 years or more; being able to read English; having internet access; and, having an operational email account.

Invitations seeking ‘seed’ participants were circulated to various national disability organisations serving members with a range of impairments that lead to wheelchair use, including the New Zealand Spinal Trust, the Earthquake Disability Leadership Group, the Multiple Sclerosis Society of New Zealand, the Cerebral Palsy Society of New Zealand, and CCS Disability Action. People expressing interest in being seeds contacted the researcher, who confirmed eligibility and then sent a recruitment code and a link to the survey website. Once a participant completed the survey, they were thanked and emailed three unique recruitment codes. Participants were asked to email one code and the survey link to three other persons they knew who were likely to satisfy the eligibility criteria. This process was envisaged to continue for multiple recruitment waves. Participation was incentivised (an entry into a draw to win an iPad); one entry for completing the survey, and another when each person they recruited completed the survey. Recruitment chains were tracked through tracing the recruitment codes. A predetermined reminder protocol was triggered when seeds or their peers failed to respond.

Results

Twelve seed participants completed the survey. A further seven participants were recruited from the second wave, from a total of four seeds, and none were recruited from the third wave. The final sample size (n=19) failed to satisfy the statistical requirements needed to reach equilibrium, the point at which the sample composition becomes independent of the initial seeds, thereby enabling the calculation of unbiased population estimates⁴. Mean age of participants was 55.6 years (range: 28–73 years), and nine were women. Eighteen were identified as New Zealand European and one was identified as New Zealand Māori. Reasons for using a wheelchair included impaired mobility resulting from spinal cord injury (n=10), cerebral palsy (n=2), spina bifida (n=2), muscular dystrophy (n=2), poliomyelitis (n=2), and arthritis (n=1). Survey completion time ranged between 7 and 36 minutes.

Discussion

Despite a rigorous recruitment process and offering incentivising participation, our use of RDS failed as an effective sampling approach amongst wheelchair users in New Zealand. There are a number of possible explanations as to why this occurred. The target population of the study was novel compared with hidden populations generally targeted by RDS studies. Research using RDS typically samples stigmatised populations, such as those with greater risk of HIV, including injecting drug users⁶, men who have sex with men^7,14, and sex workers¹⁵. Wheelchair users have experienced increased integration into many societies in recent years and are arguably less stigmatised when compared to populations traditionally sampled using RDS. Although the precise mechanism by which perceived stigma might affect RDS participation is unknown it, nonetheless, remains noteworthy. Second, the use of an unguaranteed reward (entry into a draw for an iPad) for survey completion has not been previously reported in RDS studies. This lack of guaranteed reward may have influenced participation. In addition, RDS studies often offer participants additional non-monetary free services related to the mitigation of HIV risk, such as counselling and educational material¹⁶.

Until such time as these factors, and their implications for recruitment, are better understood we feel that using RDS for recruiting wheelchairs users may have limited merit, and recommend formative research to optimise success. Exploring the areas of network characteristics, acceptability of RDS, appropriate incentive options, and seed selection have all been suggested as important for assessing the feasibility and appropriateness of RDS in certain populations¹⁵. Specifically, formative research regarding specific seed selection is warranted. Motivated seeds with large network contacts can improve recruitment effectiveness. Indeed, one RDS study exploring people who inject drugs in Sydney Australia reported that 80% of their participants resulted from one seed¹⁷.

Conclusions

Wheelchair users are an increasingly prevalent population in society who often lack an adequate sampling frame, and sampling approaches enabling valid population based estimates are becoming increasingly necessary. This paper reported the failure of RDS to survey wheelchair users. Despite the unsuccessful recruitment in this study, further research exploring the application of RDS with wheelchair users is recommended before discounting this sampling approach in this population.

Data availability

Data are available upon request from the corresponding author to protect participant identity. Demographic data will be pooled to protect participant identity, as individual-level demographic data could be theoretically traceable due to the small sample size, and suspected small national population of wheelchair users.

Consent

All participants were informed that the voluntary completion of the survey implied informed consent, including for the publication of survey data.