Before after with control

A control group is selected and observed over the entire testing period. This control group is comparable in makeup to the group that will receive the treatment. The major assumption in the design is that both groups are affected in a similar manner by extraneous factors. For this assumption to be correct, the subjects must be randomly selected and the treatments randomly assigned.

The statement of the design is:

A regional bookstore chain wants to determine the effect of new display shelves on the sale of children's books. The shelving is temporarily installed in three of its stores (test stores) and sales are compared with those in three comparable stores in which the old shelving is retained. The following sales occurred in the two sets of stores:

O3 = 290 books O2 = 350 books O3 = 285 books O4 = 320 books Effect of treatment = (350 - 290) - (320 - 285) = 60 - 35 = 25 books

The assumption is that the sales increase in the control stores (35 books) was caused by some extraneous factors. Thus, if this impact is to be subtracted from the change in sales in the test stores (60 books) the resulting value of 25 books can be viewed as the direct impact of the treatment (the new shelving).

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