Measuring and interpreting outcome is more difficult in mental health services than in some other areas of health care, for at least five reasons. First, the effect of the treatment may be to slow decline or to maintain the current level, so the score on the outcome measure itself may not improve (or may even get worse) despite best quality clinical care. Secondly, the best available evidence in the United Kingdom indicates that clinical and social variables predict no more than 30% of the variance in an individual's quality of life (UK700 Group, 1999). Thirdly, different types of outcome are desynchronous (e.g. Drury et al. 1996), changing at different rates during an intervention. Fourthly, there may not be agreement regarding what is a positive change in outcome – the patient who has fewer episodes of mania as a result of treatment may see this as a negative outcome. Finally, three levels of mental health service can be differentiated: treatment (specific interventions); programme (combination of different treatment components); and system (all programmes for a defined target group in a given area) (Burns & Priebe, 1996). The outcome data needed to evaluate each level will be very different.