Brief Summary of Chapter 9

To establish that a treatment causes an effect, you must:

It is very hard to say that the treatment is the only thing that caused the change in behavior. As Campbell and Stanley have pointed out, there are eight things other than the treatment that might cause a change in behavior.

If you try a before-after design, you have to worry that the change in behavior could be due to:

Note that history, maturation, and testing could produce real changes in your participants, whereas instrumentation, regression, and mortality could result in your "after" scores differing from your "before" scores, even though your participants didn't change at all. (See Table 9-3 or Figure 9-5) for a review.

If you compare a no-treatment group with a treatment group, the question is: Do the groups differ now because one group got the treatment--or did the two groups differ to start with? In other words, with a two-group design, you have to worry about selection. As Table 9-1 illustrates, it is very hard to eliminate the selection threat. Basically, the problem is that you can't get two identical groups of participants.

Some people naively think that matching will give you identical groups, but those people are failing to realize that:

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