After only with control

This experimental design is the one most commonly used by marketing researchers. It is intended to eliminate the influence of pretesting, so no measurements are taken between either of the groups:

A firm wanted to test the success of free gifts as a method of stimulating sales for its detergent product. It posted free sampler packages of its detergent to randomly selected homes in a neighbourhood. A month later it sent coupons offering 25 pence off the purchase of a large package of its detergent to the same group of homes. These coupons were also sent to a control group randomly selected from a comparable neighbourhood but which had not received the free sample. The coupons were coded to enable a count to be made of the number of coupons redeemed by each group. The experimental group redeemed 121 coupons and the control group redeemed 76 coupons. The difference (45 coupons) was felt to be the result of the sampler:

While this design neutralises the impact of pretest, it does not allow any measurement or analysis to be made of the total change that occurred, a situation handled well in the before-after with control design.

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