The study of hard-to-reach populations presents significant challenges. Typically, a sampling frame is not available, and population members are difficult to identify or recruit from broader sampling ...

Pew Research Center designed this study to assess the current state of online survey sampling methods – both probability-based and opt-in – and determine their accuracy on general population estimates ...

This is a preview. Log in through your library . Abstract Background: Estimating the sizes of populations at highest risk for HIV is essential for developing and monitoring effective HIV prevention ...