Much attention is paid in research publications to the methods of statistical analysis. Research design, in contrast, receives less consideration, despite the fact that it is critical; if the research design is poor, no amount of complex statistical analysis can extract useful information from the data collected. In this article, we focus on the selection of subjects for medical research studies, and outline several sampling strategies and their implications for statistical analysis. As a high level overview of sampling in medical research, we do not go into deep technical detail. Further, we discuss how to obtain a sample, but not how large the sample should be; we refer readers to Lachin1 for an introduction to the important topic of sample size and statistical power calculations.