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Sequence Effects
Sequence effects are potential confounding influences in experiments where subjects are exposed to multiple conditions. Sequence effects refer to potential interactions among conditions of an experiment based on the sequences these treatments are presented. Sequence effects are distinct from order effects, where the actual order of conditions influences the outcome, and carryover effects, where subjects are permanently changed by the manipulation. To illustrate the differences among sequence, order, and carryover effects, imagine an experiment where subjects are asked to pick up and guess the weight of different objects on multiple daily sessions. A sequence effect would be the perceived weight of a given object being influenced by whether a light or heavy object was handled just before. In contrast, an order effect would be the perceived weight of objects increasing as the experimental session progresses and subjects grow fatigued. Finally, a carryover effect would be the perceived weight of objects decreasing across sessions as subjects grow stronger from all the excessive lifting.
Controlling for Sequence Effects
Multiple methods are available for controlling for sequence effects. Ideally, sequence effects can be controlled for within individual subjects. Failing that, experimenters will need to control for sequence effects across subjects.
Controlling for Sequence Effects within Subjects
Sequence effects can be controlled for by counterbalancing experimental conditions within subjects, where each subject is exposed to every possible combination of experimental conditions. This is only practical when there are relatively few experimental conditions. A more restrictive means of counterbalancing experimental conditions within subjects is to use reverse counterbalancing, where experimental conditions are presented first in one order and then again in reverse order. For example, suppose participants are asked to rate the intensity of a bright, a medium, and a dim light stimulus after placebo or a drug treatment suspected to cause mild light sensitivity. The experimenter suspects that viewing the bright light makes the next light viewed seem less intense, and conversely, viewing the dim light makes the next light viewed seem more intense. To control for these influences, the researcher first presents the stimuli in the order of bright, medium, and then dim, and after a break presents the stimuli in the reverse order of dim, medium, and then bright. Then, the researcher will average intensity ratings obtained for each stimuli during the two orders.
Controlling for Sequence Effects between Subjects
If counterbalancing within subjects is not possible or not practical for some reason, another strategy is to counterbalance conditions between subjects. Suppose the investigator in the previous example deemed it necessary to test all six possible combinations of the three light stimuli. In this case, it would be possible to present each of the six different orders to individual subjects, match these orders across placebo and drug conditions, and compare the average stimuli intensity ratings after placebo and drug treatment. In experiments examining more conditions, it becomes more practical to control for sequence effects using a balanced Latin square. In a balanced Latin square, each condition is immediately preceded once by all other conditions. A balanced Latin square cannot control for sequence effects as completely as counterbalancing; however, it does offer a reasonable degree of control with far fewer subjects.
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