Up to 5-15% of clinical encounters lead to diagnostic errors, i.e. delayed, incorrect or missed diagnoses. [1] The mortality, morbidity and cost of these errors are considerable; [2-5] despite 40 years of technological advances they remain largely unchanged. [6] Strategies to address this major cause of patient harm must identify healthcare professionals at increased risk of making errors, characterise the errors they make and provide targeted, evidencebased intervention. [1] Taxonomies of the 'root' causes of diagnostic errors have been developed with a view to error reduction and remediation. [1,7,8] Graber et al. [1] identified three types of diagnostic errors (no-fault, system and cognitive) and reported that cognitive and system factors contributed to diagnostic errors. They clustered the root cognitive contributions to diagnostic errors (CCDEs) in four categories: faulty knowledge, data-gathering errors, data synthesis difficulties and failed verification of the data used to make the diagnosis. Schiff et al. [7] categorised errors according to the phase of the patient consultation process: access/presentation to healthcare, patient-practitioner encounter (history and physical examination), ordering and interpreting tests, making a diagnosis (assessment) and further consultation or referral and follow-up. Retrospective studies using this taxonomy have found that practitioner-patient encounters (history and physical examination), ordering and interpreting of tests and making a diagnosis (assessment) contributed most to errors. [3,8-10] Most of these studies were conducted in mixed populations of healthcare professionals [1,3,9,10] and did not focus on residents who are known to be at increased risk of making medical errors. [11] Two studies of residents showed that both cognitive and system factors contributed to diagnostic errors. [12,13] These studies of malpractice claims or self-reported data are, however, >10 years old and did not focus on characterising CCDEs. Furthermore, their retrospective design limits the accuracy of the data owing to hindsight and outcomes biases, incomplete patient records, variable reviewer reliability and uncertainty about the final diagnoses made. [4,7,14] Prospective studies characterising CCDEs that residents make in patient consultations are needed to better align current training needs and remediation efforts. A central part of the diagnostic process is data gathering, i.e. taking a history and performing a physical examination of the patient. While a thoroughly conducted history and physical examination can lead to an assessment in at least 60% of cases, [6,15,16] errors related to these contribute to diagnostic errors in up to 61% of cases. [1,3,8-10] As summarised by Feddock, [17] the variable clinical competence of trainees [18,19] may be ascribed to many factors, including progressive decline in bedside teaching, limited direct observation during real patient encounters, and limited feedback regarding clinical skills and performance in the workplace. Knowledge of clin...