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When general population samples are assessed on two occasions with psychiatric symptom or personality measures, a mean change in scores towards less psychopathology is often observed. This re-test artefact is a potential threat to the validity of longitudinal studies. Data from a longitudinal general population study were analysed to discover under what circumstances the re-test artefact occurs. It was found that the artefact is unrelated to the time lag between occasions, being equally strong at intervals ranging from 4 to 34 weeks. However, the artefact did not occur for all measures, but was confined to those assessing negative self-characteristics and administered orally by an interviewer. These findings are consistent with both the 'mechanical responding' and 'social desirability' hypotheses of the re-test artefact.
Jorm et al. (Mon,) studied this question.