In within-subjects designs, participants are exposed to every experimental condition. In such a design, the participant serves as his or her own control. Performance is compared within individual participants. Order and sequence effects are major sources of error in within-subjects designs.

Order effects produce changes in performance based on the order of the condition in the experiment and not the manipulation in the specific condition. Practice effects can be considered order effects. In cognitive experiments, performance is usually lower on the first task because participants are unfamiliar with the setting. Once the participants become familiar with what is required, performance increases.

Fatigue effects are also order effects. Performance is worse in later conditions because participants are tired. Sequence effects are produced by characteristics of the experimental manipulation. For example, in a study of perception of weight, participants will judge a weight lighter if it follows a heavy rather than a light weight. The same occurs in reverse; participants will judge weights as heavier if they follow a light rather than a heavy weight. Sequence effects are caused by an interaction between order and specific aspects of the manipulation.