Order Effects Definition
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.
Types of Order Effects
Order effects occur for many reasons. Practice effects occur when participants warm up or improve their performance over time. In reaction time studies, for example, participants usually respond faster as a result of practice with the task.
Participants may also perform differently at the end of an experiment or survey because they are bored or tired. These fatigue effects are more likely when the procedure is lengthy and the task is repetitive or uninteresting.
Carryover effects occur when the effect of an experimental condition carries over, influencing performance in a subsequent condition. These effects are more likely when the experimental conditions follow each other quickly. They also depend on the particular sequence of conditions. For example, people’s estimates of height may be lower after they have been exposed to professional basketball players than after they have been exposed to professional jockeys.
Interference effects occur when previous responses disrupt performance on a subsequent task. They are more likely when the second task quickly follows the first and the response required in the second task conflicts with the response required in the first task.
Ways to Control Order Effects
Researchers use a variety of methods to reduce or control order effects so that they do not affect the study outcome. The choice depends on the types of effects that are expected. Practice effects can be reduced by providing a warm-up exercise before the experiment begins. Fatigue effects can be reduced by shortening the procedures and making the task more interesting. Carryover and interference effects can be reduced by increasing the amount of time between conditions.
Researchers also reduce order effects by systematically varying the order of conditions so that each condition is presented equally often in each ordinal position. This procedure is known as counterbalancing. For example, with two conditions, half of the participants would receive condition A first followed by condition B; the other half would receive condition B first followed by condition A.
Sometimes there are so many possible orders that it is not practical to include all of them in a study. Researchers may then present the conditions in a different random order for each participant, or they may include a subset of the possible orders.
- Shaughnessy, J. J., Zechmeister, E. B., & Zechmeister, J. S. (2006). Research methods in psychology. New York: McGraw-Hill.