5. Independent and Dependent Sampling

It’s important at this time to distinguish between sampling methods that result in an independent sample and methods that result in a dependent sample. Two (or more) samples are called independent if the members chosen for one sample do not determine which individuals are chosen for a second sample. Two (or more) samples are called dependent if the members chosen for one sample automatically determine which members are to be included in the second sample.

Dependent samples are often called matched-pairs samples because individuals in one sample can be “matched” with a corresponding individual used in another sample. The terminology is self-evident because we’re going to observe a “pair” of individuals, one involved in each sample, which will allow us to compare results for the given pair. For example, a husband and wife are often considered a pair, with the husband in one sample and the wife in the other. We then compare the difference(s) in characteristics of interest. Twins also make a great pair. The term “pair” can be misleading because sometimes the pair is only one individual who is involved in both samples. This often arises in “before and after” experiments.