Psychiatrists, psychologists, and nursing staff are increasingly making direct observations and ratings of ward behaviour. Characteristically, a nurse may be asked to complete a multi-item rating scale on a group of patients during the course of a drug trial. Several factors are involved in the choice of an appropriate scale for a particular purpose. Among these factors are the number of points per item, which defines the sensitivity to change of the item, and the total number of items in the scale, which affects the time taken to complete the scale and hence the frequency of rating that can be permitted in an assessment schedule.