According to BMJ Quality and Safety, the International Journal of Healthcare Improvement, half of physicians surveyed reported more than 5% of their patients were difficult to diagnose.  But, physicians with more experience had less difficulty.  Almost 20% of the physicians found to have "inadequate knowledge." Physicians are advised to slow down, take more time with patients, "think carefully," and perform "cognitive work" to reach correct diagnosis. See "Challenges of Making a Diagnosis in the Outpatient Setting: a Multi-Site Survey of Primary Care Physicians." BMJ Quality and Safety 21, pp. 641-648, 2012