Patient allocation is common practice in psychiatric nursing. This study investigates nurse aptitude, a sub-concept of competence, to validate and refine patient allocation. The focus is on operationalizing aptitude for daily management purposes. Aptitude varies in function of the tasks to be performed and the therapeutic relationship of each psychiatric nurse with her individual patients. Time, job context, socio-cultural context and semantic frame of reference have also to be taken into account when operationalizing aptitude. Self and peer opinions are a basis for aptitude measurement as patient care and its management are shared among a small team of nurses. A meticulous idio-graphic analysis is necessary. These constraints generate several measurement difficulties that are to be dealt with without too many unrealistic assumptions. The research design enabled reliable aggregation of peer ‘on’ and ‘from’ opinions. These allowed confrontation of opinions and identification of bias in these. Stepwise explorative experimentation led to gauging and calibrating a specific aptitude questionnaire to the context of psychiatric nursing. The final version is now being applied in practice in 14 wards of 11 hospitals in Flanders.