Paired Data: Definition

Paired data is where natural matching or coupling is possible. Generally this would be data sets where every data point in one independent sample would be paired—uniquely—to a data point in another independent sample. This might be because they come from the same observational unit; the same individual, or the same location. Or it might be because they come from related units: data from a husband and wife, a brother or sister, or from one individual in an experimental group and one in a control group.

Paired data must be analyzed as such; and cannot be dealt with as we would deal with independent samples.

Types of Paired Data

Paired data may include:

• Duplicate (double) measurements on the same samples, meant to account for within-subject variability.
• Sequential measurements (pre-test/post-test). Also, measuring some factor both before and after a time period passes or before and after an intervention.
• Cross-over trials: individuals are randomly assigned to one of two treatments and then afterward assigned to the second treatment.
• Matched samples: where individuals are matched on similar or identical personal characteristics, such as age and sex (this method might be used to assign each test individual a control).

Example of Paired Data Sets

If you are running a study on the average amount of time spent looking at smartphones, and work with a randomly selected sample of 10 men and 10 women, your sample is independent. But if your sample consists of 10 couples, and you compare the amount of time a husband and wife spend looking at their phones, you are working with paired data.

If you are studying the effects of massage on blood pressure, and take data from two randomly selected samples of people: one receiving daily massages, one which doesn’t, your data is independent. But if, after the initial study is complete, the same individuals are assigned to the opposite groups, and measurements are taken again, these two data sets will be paired.

References

Gerstman, Bud. StatPrimer Chapter 7: Paired Samples