Panel conditioning is an effect sometimes observed in repeated surveys when a sample unit's response is influenced by prior interviews or contacts. Various possibilities have been suggested to explain the cause. Panel conditioning can affect the resulting estimates by introducing what is sometimes called "time-in-sample bias" or "rotation group bias."

In many surveys, the household, business, or other sample unit is contacted more than once over a period of time, usually to reduce the total survey cost, to produce longitudinal estimates, or to decrease the standard error of the estimate of change in the items of interest. In various documented studies, the levels of unemployed persons, expenditures, illness, victimizations, house repairs, and other characteristics were significantly higher or lower in earlier survey contacts than in later ...