How response shift may affect the measurement of change in fatigue.

If patients experience extreme fatigue during treatment, they may judge the level of fatigue following this experience differently from how they would have judged it before. This change in internal standard is referred to as a response shift. We explored whether a response shift might have occurred in patients receiving radiotherapy (n = 199). Fatigue was assessed before and after radiotherapy. Following completion of the post-test, a thentest was administered where patients had to provide a renewed judgment of their pre-treatment level of fatigue. Response shift was assessed by the mean difference between the pre-test and thentest scores. Comparing the thentest with the pretest scores, patients retrospectively minimized their pre-treatment level of fatigue. The thentest-post-test difference was significant, whereas the conventional pretest-post-test difference was not. These results are in line with the occurrence of a response shift. Additional hypotheses regarding response shift were partially supported. It is concluded that the potentially large implications of response shift justify further research.