• errors
• diagnosis
• safety

Four recent EMJ papers address patient safety.1–4 It remains an important topic that has attracted much interest since the turn of the century. Not that quality and safety of patient care weren't always uppermost in most medical minds, but the Institute of Medicine (IOM) report To err is human: building a safer health system5 brought them into sharper focus and spawned a variety of worthwhile and, as we see, continuing initiatives. Fifteen years later, the third of the IOM quality chasm series Improving diagnosis in health care has now appeared,6 with the IOM (now the National Academies of Sciences, Engineering, and Medicine) acknowledging that it had missed the obvious, or to be fair, the less than obvious in the first report. While there are a number of antecedents to diagnostic failure, chief among them must be the clinician's thinking, reasoning, problem solving, and decision making. However, a major problem for patient safety has always been that these processes are not obvious; they are invisible. They are not unknown, but we cannot see them in the obvious way that tangible issues such as equipment failures3 can be seen. Similarly, the major steps in medicating patients are also well known and highly visible, probably accounting for why medication error was mentioned 70 times in the first IOM report, whereas diagnostic error was only mentioned twice.7 Visibility amounts to measurability, and leads to the question: how can we make the processes that underlie clinical reasoning and decision making less opaque? This is important because the single unifying theme underlying all aspects of patient safety is human cognition, and the primary output of cognition is decision making, the engine that drives all behaviours involved in patient care. Cognition is a precious resource in the emergency department,8 and we …