This paper suggests the integration of single-system and group comparison approaches in evaluating treatment effectiveness of service agencies. The basic building blocks of the advocated methodology are single-case evaluations which are intentionally designed, selected and aggregated to form an assessment on the agency level. For each intervention, data on the characteristics of treatment, therapist, problem and client are collected and correlated with outcome measures toidentify differential effectiveness. The paper reports on an assessment project which utilized this approachto evaluate task-oriented family interventions. The potential benefits of this methodology are discussed.