Our sampling strategy determines who is in our study. Once we have selected our sample, we need to place them in experimental conditions. One of the most effective general control strategies is random assignment. Random assignment controls extraneous factors because it randomly distributes personal characteristics that can influence outcome across conditions.

Factors such as intelligence, motivation, and preferred learning strategy can affect performance on any learning task. If you are studying memory, you would not want all of the participants in your experimental group to be those who learned and practiced different mnemonic strategies for improving their memory. By randomly assigning participants to experimental conditions, you hope to distribute previous learning and practice across groups so that the only factor upon which they differ is the IV.