Describe the difference between between-subjects and within-subjects experimental designs.

Between-subjects design involves different participants in each condition, while within-subjects design uses the same participants in all conditions.

In a between-subjects experimental design, each participant is exposed to only one level of the independent variable. This means that different groups of participants are tested in each condition. For example, if you were testing the effect of caffeine on alertness, you might have one group of participants drink a caffeinated beverage and another group drink a decaffeinated beverage. The advantage of this design is that it avoids carryover effects, where the experience of one condition affects the participant's behaviour in another condition. However, it requires a larger number of participants and can be affected by individual differences between groups.

On the other hand, a within-subjects design involves the same group of participants being exposed to all levels of the independent variable. In the caffeine example, the same participants would drink caffeinated and decaffeinated beverages in different sessions. This design has the advantage of requiring fewer participants and controlling for individual differences, as each participant serves as their own control. However, it can be affected by carryover effects and practice effects, where participants improve simply because they are repeating the task.

In summary, the choice between a between-subjects and a within-subjects design depends on the nature of the experiment and the potential for carryover or practice effects. Both designs have their strengths and weaknesses, and the choice between them should be made carefully, considering the specific requirements and constraints of the study.