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It has been suggested that these human errors can be further categorised into: (i) those that occur at the sharp end of care by the treating team and individuals; and (ii) those that occur at the blunt or organisational level, typically through policies, procedures, staffing and culture. These errors can be further subdivided (Table 4.1).

Table 4.1 Types of errors

		Explanation	Example
Sharp errors that occur with the team/individuals treating the patient	Mistake	Lack or misapplication of knowledge	Not knowing the correct drug to prescribe
Slip or lapse	Skills‐based mistake	Knowing the correct drug but writing another one
Violation	Deliberate action that may be routine or exceptional	Not attempting to get a drug second checked as there are no staff available
Blunt/organisational errors		Policies, procedures, infrastructure and building layout that has errors embedded	Different drugs used by different specialties and departments for same condition

It is typically found that the latent/organisational issues often coexist with the sharp errors; in fact it is rare for an isolated error to occur – often there is a chain of events that results in the adverse event. The ‘Swiss cheese’ model demonstrates how apparently random, unconnected events and organisational decisions can all make errors more likely (Figure 4.1). Conversely, a standardised system with good defences can capture these errors and prevent adverse events.

Figure 4.1 The ‘Swiss cheese’ model

Each of the slices of Swiss cheese represents barriers that, under ideal circumstances, would prevent or detect error. The holes represent weaknesses in these barriers; if the holes align the error passes through undetected with the potential to cause poor outcome and patient harm.

Reconsider the example of drug error using the Swiss cheese model. The first slice is the doctor writing the prescription, the second slice is the organisation’s drug policy, the third is the midwife who draws up the drug and the fourth is the midwife who second checks the drug.

Now consider the following: What if the doctor is relatively new to the obstetric unit and unfamiliar with the specific drugs or doses used in this situation? – their ‘slice of cheese’ has larger holes. What if the organisation has failed to develop a robust drug policy that is fit for purpose and guidelines are out of date or not easily accessed? – this second slice is considerably weakened or may even be removed completely. What if the drug is drawn up by a midwife who has just returned from a career break who is not familiar with the particular antihypertensive drug used? – their ‘slice’ has also got larger holes. Labour wards are often chronically short of staff and the midwife who performs the second drug check is distracted as they are looking after two high‐risk women in labour. Inadvertently their check is only cursory – this final slice (or barrier) is completely removed.

The end result is that multiple defences have been weakened or removed and error leading to unintentional harm is more likely. Also be aware of the different types of failure within the system: (i) latent failures include organisational error (e.g. no effective policy, out‐of‐date guidelines and inadequate staffing levels); and (ii) active failures (e.g. failure to escalate, drug errors, failure to monitor or act on deteriorating vital signs).