In educational research, an order effect occurs when the order in which research subjects participate in experimental conditions affects the outcome variable being measured. For example, a researcher may be interested in examining the relative effectiveness of two versions of an online reading program on student performance. Students in the study could first complete Program A and then complete Program B, each followed by a standardized assessment. If Program B yields greater improvement than Program A, the researcher might conclude that Program B is more effective. However, without additional evidence, this conclusion may not be valid. Improvement under Program B could have been greater for reasons other than the effects of this version of the reading program. Extraneous factors, such as increased familiarity with the ...