Objective: To explore the use of an observational, cognitive-based approach for differentiating between successful, suboptimal, and failed entry of coded data by clinicians in actual practice, and to detect whether causes for unsuccessful attempts to capture true intended meaning were due to terminology content, terminology representation, or user interface problems. Design: Observational study with videotaping and subsequent coding of data entry events in an outpatient clinic at New York Presbyterian Hospital. Participants: Eight attending physicians, 18 resident physicians, and 1 nurse practitioner, using the Medical Entities Dictionary (MED) to record patient problems, medications, and adverse reactions in an outpatient medical record system. Measurements: Classification of data entry events as successful, suboptimal, or failed, and estimation of cause; recording of system response time and total event time. Results: Two hundred thirty-eight data entry, events were analyzed; 71.0 percent were successful, 6.3 percent suboptimal, and 22.7 percent failed; unsuccessful entries were due to problems with content in 13.0 percent of events, representation problems in 10.1 percent of events, and usability problems in 5.9 percent of events. Response time averaged 0.74 sec, and total event time averaged 40.4 sec. Of an additional 209 tasks related to drug dose and frequency terms, 94 percent were successful, 0.5 percent were suboptimal, and 6 percent failed, for an overall success rate of 82 percent. Conclusions: Data entry, by clinicians using the outpatient system and the MED was generally successful and efficient. The cognitive-based observational approach permitted detection of false-positive (suboptimal) and false-negative (failed due to user interface) data entry.