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What Is a Sample? 

What Is a Sample?
What Is a Sample?

Katherine M. Keyes

and Sandro Galea

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date: 23 September 2020

This chapter introduces step three: take a sample. It reviews two approaches to sampling: random sampling and purposive sampling. Simple random sampling includes quantifying variability; purposive samples are used when questioning whether a particular exposure is a cause of a health indicator. Researchers require purposive sampling to reconceptualize population individuals as quanta of person time. The chapter covers the fundamentals of epidemiologic study design including following samples free of the health indicators forward in time, determining when cases arise (cohort studies), sampling individuals at a particular point in time, assessing the burden of the health indicator (cross-sectional studies), and sampling individuals with and without the health indicator, assessing history of potential exposures (case-control studies). Methods include different ways to look at a dynamic population to capture pieces for epidemiologic study.

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