The topic of this chapter—collecting data—centers on deceptively simple questions: how should researchers select the observations to include in their studies, and how should we evaluate their choices?

Just as we saw in Chapters 2 and 3, the answers to these questions turn not on anecdotes, guesses, or impressions but rather on rules or best practices. We describe many in the course of discussing the four topics that are central to the task of collecting data: (1) Identifying the Target Population, (2) Locating or Generating Data, (3) Deciding How Much Data to Collect, and (4) Avoiding Selection Bias.

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