Well conducted research begins by articulating a clear, discrete question of interest. One of the first steps in generating RWE to address the research question is to identify a fit-for-purpose RWD source. In assessing potential data sources, consideration should be given to the PICO of interest – i.e., the Population [or Problem or Patient], Intervention [or Exposure], Comparator [or Control or Comparison], and Outcome. Specifying the PICO helps elucidate the care settings in which the patients, exposure, comparator, and outcomes are likely to be identified, informing decisions about the type and source of data likely to be relevant and valid for the research question. After selection of a fit-for-purpose data set, a well-defined study question in the PICO can help guide the selection of appropriate study design, approaches for assessing key study variables, and methods for addressing bias in RWE studies, which are the subject of Chapters 2 and 3.