As we have just described, it’s crucial for the experimental and control group procedures to be as similar as possible—differing only in the experimental manipulation itself. It’s also essential for the two groups of participants to start out the procedure being well matched to each other. In other words, there should be no systematic differences between the experimental and control groups when the experiment begins. Then, if the two groups differ at the end of the experiment, we can be confident that the difference was created during the experiment—which, of course, is what we want.
How can we achieve this goal? The answer is random assignment—the process of using some random device, like a coin toss, to decide which group each participant goes into. According to some descriptions, this is the defining element of a true exper-iment. Random assignment is based on the simple idea that people differ from each other. Some people are anxious and some are not; some like to race through tasks while others take their time; some pay attention well and others are easily distracted. There’s no way to get around these differences—but with random assignment, we can be confident that some of the anxious people will end up in the experimental group and some in the control group; some of the attentive people will end up in one group and some in the other. Random assignment doesn’t change the fact that participants differ from one to the next, but this procedure makes it very likely that the mix of par-ticipants in one group will be the same as the mix in the other group. As a result, the groups are matched overall at the start of our experiment—and that’s exactly what we want.
Notice that we’ve now solved the concerns about cause and effect. Thanks to random assignment, we know that the groups started out matched to each other before we introduced the experimental manipulation. Therefore, any differences we observe in the dependent variable weren’t there before the manipulation, and so they must have arisen after the manipulation. As we mentioned earlier, this is just the information we need inorder to determine which variable is the cause and which is the effect.
Random assignment also removes the third-variable problem. The issue there was that the groups being compared might differ in some regard not covered by the variables being scrutinized in our study. Thus, students who take Latin in high school might also be more motivated academically, and the motivation (not the Latin) might be why these students do especially well in college.
This problem wouldn’t arise, however, if we could use random assignment to decide who takes Latin classes and who doesn’t. Doing so wouldn’t change the fact that some students are more motivated and others are less so; but it would guaran-tee that the Latin takers included a mix of motivated and less motivated students, and likewise for the group that does not take Latin. That way, the groups would be matched at the start—so if they end up being different later on, it must be because of the Latin itself.