Because different people participate in each condition, could this explain any differences in the outcome?

• Random assignment to conditions creates equivalent groups, on average.

In order to make the causal inference that an apology causes people to desire less revenge, one important feature must be present in the experiment. Participants must be randomly assigned to the conditions (i.e., the scenarios) of the experiment. Random assignment means that a random procedure, such as a flip of a coin, determines which condition each participant experiences. In clinical psychology research, random assignment is particularly important in studies evaluating the effects of a particular psychological treatment. It is essential that participants are randomly assigned to either receive the treatment or to a condition that involves no treatment, an alternative form of treatment, or a placebo. Chapter 15 of your clinical psychology textbook provides a detailed discussion of issues in treatment research in clinical psychology

Because different groups of people participate in the different conditions of the experiment, an alternative explanation for the outcome (i.e., mean revenge ratings of 4.0 and 8.0) is that the people in the two groups differed in terms of whether they are naturally more vengeful or forgiving. That is, the mean revenge ratings might differ because different people participated in the groups of the experiment, not because of the presence or absence of an apology.

The solution to this potential problem, though, is random assignment. Random assignment creates equivalent groups of participants, on average, before participants read the scenarios. Neither group is more vengeful or forgiving; nor do the groups differ, on average, in terms of any other potentially important characteristics. Therefore, we can rule out the alternative explanation that differences in revenge might be due to characteristics of the people who participated in each group.

Isn’t it possible that conditions of an experiment differ in other ways, besides the independent variable?