A number of texts analyse the advantages and limitations of single-case designs. The ultimate goal of all forms of single system designs is to improve practice. The key advantages—the designs provide a model for enhancing the accountability of practitioners and single-case evaluation is one methodology that can be used alongside others, and provides an empirical dimension to the process of evaluation. Systematic, consistent use of single system designs will allow practitioners, and agencies, to collect a body of data about the effectiveness of practice that provides more or less objective information about the success of our practice. The use of single-case designs presupposes the application of valid and reliable measuring instruments. The limitation had been to concentrate on operationalising target problems rather than on defining intervention programmes. The main purpose identified, based on the needs of practice at the time, was for an effectiveness strategy that could enable a systematic tracking of client outcomes.