# Statistics and Probability Dictionary

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### Matched Pairs Design

 Pair Treatment Placebo Vaccine 1 1 1 2 1 1 ... ... ... 499 1 1 500 1 1

A matched pairs design is a special case of a randomized block design. It can be used when the experiment has only two treatment conditions; and subjects can be grouped into pairs, based on some blocking variable. Then, within each pair, subjects are randomly assigned to different treatments.

The table to the right shows a matched pairs design for a hypothetical medical experiment, in which 1000 subjects each receive one of two treatments - a placebo or a cold vaccine. The 1000 subjects are grouped into 500 matched pairs. Each pair is matched on gender and age. For example, Pair 1 might be two women, both age 21. Pair 2 might be two men, both age 21. Pair 3 might be two women, both age 22; and so on.

For this hypothetical example, the matched pairs design is an improvement over a completely randomized design. Like the completely randomized design, the matched pairs design uses randomization to control for confounding. However, unlike the other design, the matched pairs design explicitly controls for two potential lurking variables - age and gender.