Table 1. Factors and levels included in the vignette experiment on recruiting participants via web-based environments for health-related surveys and blood tests. Directly after each vignette, respondents were asked: “How likely would you take part/agree to be tested in the survey/blood test in such a situation?” For the majority, the response category was on a 7-point rating scale, while for a minority of respondents, it related to a 5-point rating scale. Regarding the 7-point scale, response categories were as follows: very likely (7), likely (6), somewhat likely (5), neither likely nor unlikely (4), Somewhat unlikely (3), unlikely (2), and very unlikely (1).

Dimension and level		Wording
Topic
HIV		Among other health issues the survey will ask questions about HIV. OR The blood test is used to assess the prevalence of HIV in your country.
Diabetes		Among other health issues the survey will ask questions about Diabetes/high level of blood sugar. OR The blood test is used to assess the prevalence of Diabetes/high level of blood sugar in your country.
Sponsor
NGOa		Imagine that the survey is conducted by an NGO.
Statistical office		Imagine that the survey is conducted by the National Statistical Office.
Health ministry		Imagine that the survey is conducted by the Ministry of Health.
Local university		Imagine that the survey is conducted by a local university.
Incentive
None		You would receive US $0 for your participation in this survey.
US $2 cash		You would receive US $2 for your participation in this survey.
US $2 lottery		You would receive a 1% chance to win US $200 in a lottery for your participation in this survey.
US $2 voucher		You would receive a phone voucher worth US $2 for your participation in this survey.

^aNGO: nongovernmental organization.