This study identified 100 cases of diagnostic error in internal medicine and conducted retrospective chart reviews and interviews to determine what role systems-related and cognitive factors played. Investigators discovered nearly six independent factors per case and provide a detailed discussion of the most common findings. These include system-related problems such as inefficient processes and cognitive problems such as failure to consider alternative clinical diagnoses. The authors propose a classification system for the factors that produce errors and offer a number of strategies to reduce their incidence.