Random
Assignment
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.
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