This paper discusses several methods of estimating task difficulty of onscreen object movement tasks for three subject groups using five standard computer input devices (touchscreen, mouse, keyboard, trackball, and locking trackball). The subject groups are as follows: 19 computer-experienced adults, 39 normally developing children, and 15 children with mental retardation. Three separate measurements of cognitive load (mastery, speed, and user characteristics) are reported between user groups to compare conclusions about input device difficulty across different measurement techniques. Results indicate that speed measurements tend to emphasize similarity in device efficiency for skilled users, but that mastery measurements are essential to represent device effectiveness, particularly for unskilled users. Cognitive as well as physical user characteristics were associated with the successful acquisition of device control skills with training. Because each of the measurements used separately result in different conclusions about cognitive load, a combination of measurement techniques would be essential to select appropriate computer-based assistive technology.