Читать книгу Health Psychology - Michael Murray - Страница 113

The cross-over or within-participants design is used when the same people provide measures of a dependent variable at more than one time and differences between the measures at the different times are recorded. An example would be a measure taken before an intervention (pre-treatment) and again after the intervention (post-treatment). Such a design minimizes the effect of individual differences as each person acts as his/her own control.

There are a number of problems with this design. Any change in the measure of the dependent variable may be due to other factors having changed. For example, an intervention designed to improve quality of life among patients in a long-stay ward of a hospital may be accompanied by other changes, such as a new set of menus introduced by the catering department. In addition, the difference may be due to some aspect of the measuring instrument. If the same measure is being taken on both occasions, the fact that it has been taken twice may be the reason that the result has changed.

Failure to find a difference between the two occasions doesn’t tell you very much; in a worsening situation, the intervention still might have been effective in preventing things from worsening more than they have already. The counterfactual scenario in which nothing changed is an unknown entity.