What are order effects?
From PsychWiki - A Collaborative Psychology Wiki
In a repeated measure design, the effects that the order of presenting the treatments have on the dependent variable. Order effects that are associated with the passage of time include practice effect (improvement in performance due to repeated practice with a task) and fatigue effect (decline in performance as the research participant becomes tired or bored while performing a sequence of tasks) (Cozby, 2009).
Example: This fictitious study focuses on the effects of proactive inhibition (PI) on recall in short-term memory using a 3 (PI) X 5 (trials) repeated measures design to test the three different conditions of PI (PI/No Release, PI/Release, and No PI control) and five trials as independent variables with the dependent measure being the mean score of the correctly recalled stimulus words. Recall increased dramatically in the PI/Release condition in the fifth trial.
Application: The study confirms that recall increased dramatically in the PI/Release condition in the fifth trial not because of the order of presentation of treatments (order effect) or practice gained on prior tasks (practice effect) but because exposure to words from a new semantic group were presented in that trial.
Cozby, P. C. (2009). Methods of Behavioral Research: Tenth Edition. New York, NY: McGraw-Hill.