A further test of Foulds' personality and personal illness differentiae in a psychiatric group.

Test and retest scores on the Symptom Rating Test (SRT), Manifest Anxiety Scale (MAS) and Maudsley Personality Inventory (MPI) were obtained from 45 non-psychotic psychiatric in-patients. The change in scores and the score distributions were examined to assess the extent to which these tests meet Foulds' criteria for measures belonging respectively to the universes of personality and personal illness. MPI Extraversion emerged as a stable, normally distributed, personality trait in contrast to the SRT which showed the characteristics of a symptom-state measure. MPI Neuroticism and MAS scores could not be allocated definitively to either universe and seemed to be hybrids. It is suggested that more attention be paid to the 'purity' of scales if meaningful interpretation is to be made in treatment assessment.