1. in within-subjects designs, the influence of the order in which treatments are administered, such as the effect of being the first administered treatment (rather than the second, third, and so forth). As individuals participate in first one and then another treatment condition, they may experience increased fatigue, boredom, and familiarity with or practice with reacting to the independent variable. Any of these conditions could affect the participants’ responses and confound the results of the study. Researchers often use counterbalancing to control for order effects. See also carryover effect; sequence effect.

2. the influence of the order in which items or statements are listed on surveys and questionnaires. Three basic types of question order effect have been identified: (a) unconditional, in which the answer to a subsequent question is affected by the individual having responded to the prior question but not by the response given on that prior question; (b) conditional, in which the answer to a subsequent question depends on the response given to the prior question; and (c) associational, in which the correlation between the prior and subsequent questions changes depending on which is asked first. Order effects make it difficult to know whether change over time reflects legitimate respondent differences or question effects.