What are the strengths and weaknesses of a post-only, pre-post and test-control design for measuring persuasion in copy testing?
There are three main approaches for measuring copy persuasiveness. In the pre-post design, the respondent is asked about attitudes or dispositions towards the brand before being exposed to the ad stimulus and then again after being exposed to the stimulus. The difference in levels pre- and post-exposure is the measure of persuasion. In the exposed-unexposed design, persuasion measures are taken from a sample that has not been exposed to the advertising and from a matched sample that has been exposed. The difference in levels between the unexposed (control) and exposed (test) cells is the measure of persuasion. In the post-only design, attitudes are obtained after exposure and compared to norms based on results from previous studies of similar advertising. Each of the approaches has its strengths and weaknesses. In the pre-post design, because the measurement is being taken among the same person before and after stimulus exposure, sampling error is low and so more powerful conclusions c