The movement toward evidence-based interventions (EBI) in school psychology places new expectations on scientist-practitioners for evaluating intervention effectiveness (Kratochwill & Stoiber, 2000, 2002). Many such interventions in the school context are conducted for individual students, requiring single case designs. In this paper we provide school psychologists with a technique for assessing intervention effectiveness within complex (multiphase) single case designs, including multiple baseline designs. Phase contrasts can provide the researcher with one or more effect sizes which together summarize the elements of a multiphase design. Phase contrasts can be flexibly specified to match expectations for client performance during particular phases. Additional topics addressed are multiple baseline designs, confidence intervals around effect sizes, and auto-correlation.