On-line performance assessment was developed to maximize the usefulness of performance assessment and to minimize the time and labor costs incurred. This paper reports on the development of an on-line performance assessment instrument, focusing on the establishment and validation of the scoring rubric and its implementation in the Rasch model, the training of raters, and technological implementation of the instrument. The instrument discussed is the Assessment of Motor and Process Skills (AMPS), a performance instrument used by occupational therapists to determine the effectiveness of a program of rehabilitative therapy for clients. Clients complete skills from the assessment and are rated for difficulty and level of challenge they can handle. A computerized scoring version was developed with software that depends on calibrations of task and skill items developed from previously collected AMPS data. Approximately 3,300 clients were used in the initial study, and data from 4,766 clients were used in the final calibration. The resulting instrument is a highly portable and user-friendly program that helps therapists manage therapeutic regimes for their clients. Approximately two-thirds of the paper consists of appendixes containing the program user manual and sample reports. (Contains one table and five references.) (SLD)