This exploratory study investigated two factors believed to influence the efficiency (cost/time) and effectiveness (accuracy/validity) of functional assessment, client’s level of functioning, and complexity of the skill items. The results show that cost depends on the mode of assessment; criterion testing was found to be more than twice as expensive as interviewing the client’s caretakers. The effectiveness or accuracy of the assessment was found to be enhanced under two sets of circumstances: (a) when clients were functioning at a higher level and (b) when assessment items were less complex. The findings have implications for developing client assessments that are both efficient and effective.