In order to deliver good quality of care a correct diagnosis based on complaints and symptoms of a patient is required. Physicians arrive at a diagnosis based on the patients' history, a physical examination and additional information such as results of laboratory tests and imaging techniques. From a large amount of information the physician has to select the information relevant for diagnosing the patient correctly. Based on this information different diagnoses are considered, investigated and subsequently confirmed or ruled out. Sometimes, this is a relatively simple task while in other cases it is complicated and a lot of decision making skills are required. International studies demonstrate that a substantial percentage of the diagnoses are incorrect [
1,
2]. The percentages found in different studies vary widely [
2,
3]. Berner & Graber (2008) [
4] show that the rate of diagnostic error depends on medical specialty. They summarize that in perceptual specialties (e.g. pathology and radiology) diagnostic errors appear in 2–5%, while in other specialties it occurs in 10–15% of the cases. In several post-mortem studies the percentage increases to 40% [
5].
An incorrect diagnosis can have major effects on treatment and therefore on the outcome of treatment [
6] and can even lead to a patient's death. The mortality caused by diagnostic failure is higher than for any other type of medical error [
7,
8]. Patients are also aware of diagnostic errors as research shows that 30% or more of the malpractice claims are about diagnostic adverse events [
9‐
12]. It is also known that at the emergency department, out of all medical errors, patients are most concerned about being misdiagnosed [
13]. For both patients and physicians diagnostic error is important. However, studying diagnostic error is difficult.
Existing research on diagnostic error can be divided into two main research areas. The first area, post mortem studies and retrospective adverse event studies [
2,
3,
14,
15], focuses on the adverse outcomes and count and describe incorrect diagnoses and study the adverse outcomes for the patient. Another field of research focuses on cognitive error in the diagnostic process [
16,
17]. According to the model of unsafe acts, cognitive errors can occur due to intended or unintended actions [
18]. Unintended actions are slips and lapses and are described as errors which result from a failure in the execution and/or storage of an action sequence, regardless of whether or not the plan which guided them was adequate to achieve its objects [
18]. An example of a slip is applying for an incorrect test due to checking the wrong box on the laboratory form. The intended actions are mistakes and violations. Mistakes occur when a plan to perform an action is inadequate to achieve its intended outcome [
18], for example when a physician applies an incorrect laboratory test while he thought it was the correct test to rule out a certain disease. Violations are defined as any behavior that deviates from accepted procedures, standards and rules such as not applying a test while knowing it would be better to apply for the test. Since slips and lapses occur in the execution phase of a task, they are more influenced by high workload and fatigue [
19‐
21] resulting for example in distraction from a routine event. Since mistakes are errors in knowledge and interpretation, they are likely more influenced by work experience [
18]. Most cognitive errors do not lead to adverse outcomes. However, the relationship between cognitive error in diagnostic reasoning, circumstantial factors (workload, fatigue and work experience) and the occurrence of adverse events has not yet been studied.