Objective: Occupational therapists frequently evaluate instrumental activities of daily living (IADL) performance with interviews and observation of simulated tasks. This study examined the congruence of judgments of independence when comparing task performance assessed by interview and simulation with task performance assessed by observation in the natural environment.

Method: Twenty person with severe mental illness were selected through convenience sampling and evaluated on two IADL task (making a purchase in a store and using the bus). The participants were evaluated on each task with two methods of assessment: interview and simulation and observation in the natural environment.

Results. Results indicated inconsistent performance across assessment approaches and tasks and supported the importance of considering contextual features in understanding the complexity of performance in the natural environment. A trend toward false positives was found in which several participants were judged independent on the IADL assessment but could not perform the same tasks in the natural environment.

Conclusion. Occupational therapists should be cautious when making judgments of independence on the basis of interview and observation of simulated tasks. Evaluating IADL performance in the persons’ natural environment may provide more accurate information.