What are carry-over effects?
|A carryover effect is an effect that "carries
over" from one experimental condition to another. Whenever subjects perform
in more than one condition (as they do in within-subject designs) there is a
possibility of carryover effects. For example, consider an experiment on the
effect of rate of presentation on memory. Subjects are presented with a list
of words and asked to recall as many words as they can. In one condition,
the words are presented one word per second; in the other condition, the
words are presented two words per second. The question is whether or not
having performed in one condition affects performance in the second
condition. Perhaps learning the first list of words will interfere with
learning the second list because it will be hard to remember which words
were in each list. Or maybe the practice involved learning one list will
make it easier to learn a second list. In either case, there would be a
carryover effect: performance on the second list would be affected by the
experience of being given the first list. Assume two experimental conditions:
A1 and A2.
the subjects perform in Condition A1 first and then in A2;
the other subjects perform in Condition A2 first and then in A1.
The following table of means shows "symmetric carryover effects."
Condition Order A1 A2 A1 given first 8 A2 given first 5 A1 given second 10 A2 given second 7
First notice that performance in Condition A1 is three points better than performance in Condition A2.
Now consider the carryover effects by comparing the performance in Condition A1 when A1 was given first with performance in Condition A1 when A1 was given second. The table shows that performance in Condition A1 was two points better when it was given second (10) than when it was given first (8). Now, notice that the carryover effect is the same for Condition A2: Performance in A2 was two points better when it was performed second (7) than when it was performed first (5). Therefore, performing in Condition A1 prior to performing in Condition A2 raises performance in Condition A2 by two points. Analogously, performing in Condition A2 prior to performing in Condition A1 raises performance in Condition A1 by two points. The carryover effects are thus symmetric.
Carryover effects can also be asymmetric. Consider the following table of means.
Condition Order A1 A2 A1 given first 8 A2 given first 5 A1 given second 10 A2 given second 15