Purpose: To develop a taxonomy of patient-safety events in family medicine based on a theoretical model of human cognition.

Methods: Observations, staff interviews, task analysis, and confidential reporting of patient-safety events by staff in five clinics. Fault Trees were used to analyse the reports. Taxonomy development was an iterative process, both theory and data driven.

Results: There are three levels of classification: 1. Cognitive domain and psychological mechanism. The information processing framework is used to classify errors according to the ‘cognitive domain' that failed: perception, situation assessment/response selection, memory, response execution. The psychological mechanisms aim to explain how the cognitive domain failed: expectancy, loss of attention, interference, false assumption, cognitive bias. 2. Immediate internal causes. These are affective and cognitive states that influence cognition through the psychological mechanisms: lack of knowledge, alertness/fatigue, cursory/hurried work approach, inflexible application of procedures, stress/preoccupation. 3. Performance-Shaping Factors (PSFs). These are conditions that predispose to error and are divided into organisational and technical, each with a number of subcategories.

The taxonomy was used to classify 77 reported events. A tendency not to report self-made errors, generally found in reporting schemes, made it difficult to extract cognitively-related information. The cognitive domain was identified in 46 reports. Most errors related to situation assessment (45%), often caused by false assumptions (64%). Response execution failures (24%) were most often due to interference/confusion (81%). Perception failures (15%) were most often due to expectancies of visual stimuli (86%). All memory failures involved forgetting intentions (15%) and were all due to losses of attention. PSFs were almost always mentioned in the reports: task demands (37%), task fragmentation (24%), communication (12%), information presentation (11%), and information availability/delays (9%). Fragmentation and task demands were most frequently associated with errors in situation assessment and memory. Time pressures and information presentation were associated with perceptual errors. Most execution errors happened in situations of cue similarity or proximity.

Conclusions: The taxonomy's comprehensiveness and reliability remain to be tested. Results suggest that task fragmentation, excessive task demands, and the availability and presentation of information are important factors in safe clinical practice. Their effects on cognition can be identified and ascertained across events in a database that maintains the causal links between the individual, the task, and the environment.