Rasch analysis of the School Function Assessment provides additional evidence for the internal validity of the activity performance scales.

OBJECTIVE The internal construct validity of the School Function Assessment (SFA) was determined by examining its unidimensionality and hierarchical structure. METHOD The SFA was completed for 64 elementary school children (35 with disabilities, 29 without disabilities). Data were examined through Rasch analysis. RESULTS The majority of test items (252 of 266) within the Activity Performance Scales met the criterion set for Rasch goodness-of-fit statistics. All but three scales were found to be unidimensional, measuring a single construct. Item difficulty analysis yielded a hierarchical structure of the scales similar to the existing item layout of the SFA. CONCLUSION Internal validity was supported for 15 of the 18 Activity Performance scales. Each scale is psychometrically sound in measuring a specific functional task. The reliable hierarchical pattern of the assessment helps anticipate and document the student's progress in functional performance from easier to more difficult school tasks.