Order effects refer to differences in research participants' responses that result from the order (e.g., first, second, third) in which the experimental materials are presented to them. Order effects can occur in any kind of research. In survey research, for example, people may answer questions differently depending on the order in which the questions are asked. However, order effects are of special concern in within-subject designs; that is, when the same participants are in all conditions and the researcher wants to compare responses between conditions. The problem is that the order in which the conditions are presented may affect the outcome of the study.

Order effects occur for many reasons. Practice effects occur when participants warm up or improve their performance over time. ...