Carry-Over Effect (Carryover Effect)
In research a carry-over or carryover effect refers to any lingering effects of a previous experimental condition that are affecting a current experimental condition.
Essentially it is an effect that "carries over" from one experimental condition to another. This effect is seen when a subject performs in more than one condition making this is a common concern in within-subjects design.
For instance, a researcher wants to know the effects of a medication on memory. The subject is given a list of words to memorize in two different conditions: with a placebo and with the real medication. The researcher doesn't consider the possibility of carryover effects and uses the same list of words for both experimental conditions.
Any significant results of improved memory may not be a result of the medication - the improved memorization could have been carryover effects from the first condition. By using the same list from the first condition the subject is more likely to recall the words the second time. The carryover effects from the first condition are influencing the second condition making it seem as if there is significance when there is not.