Revealing response shift in longitudinal research on fatigue--the use of the thentest approach.

In this study we examined whether response shift resulting from changes in internal standards occurs in cancer patients undergoing radiotherapy. Ninety-nine newly diagnosed patients undergoing radiotherapy were administered two standardized self-report measures of fatigue prior to receiving radiotherapy. After completion of radiotherapy, patients filled out these questionnaires as a conventional posttest and in reference to how they perceived themselves as they were prior to radiotherapy (a so-called 'thentest'). A transition (direct change) score on fatigue was used as a stratification measure. Patients were subsequently interviewed about their responses. The pattern of mean scores indicative of response-shift effects was found in two distinct subgroups: patients experiencing diminishing levels of fatigue and patients facing early stages of adaptation to increased levels of fatigue. Since response shift may adversely affect the results of self-reported outcomes in clinical trials or other longitudinal research, further research is very much needed.