Participants in an experiment are given the same list of words to study for 3 minutes and then, after a delay, are asked to recall as many words as possible from the list. The length of the delay is manipulated to be 2 minutes, 5 minutes, or 10 minutes. Because different groups need different amounts of time, researchers assign the first 25 participants who arrive to the 10-minute group, the next 25 to the 5-minute group, and the final 25 to the 2-minute group. What kind of confound does this create