This paper summarizes a 12-month evaluation of an emergency assessment and community care psychiatric nursing service to demonstrate the possibility and desirability of health service providers evaluating their own service provision. The radical question central to such an assessment is: 'What would have been the case if the service assessed had not existed?' This question is more difficult to answer than might at first appear. However, recent changes in UK legislation concerning health care in general, and community care of the mentally ill in particular, make the assessment of new services of great importance. What is most needed in making such assessments is not technical expertise in data collection and analysis but non-specialized critical acumen in making inferences from data routinely collected in the course of providing any service. This is illustrated using the example of a community psychiatric nursing service which afforded an opportunity to consider the impact of a new project on established services.
