This survey shows that among quality improvement professionals, the meaning of the Swiss cheese model of medical error is far from univocal. On average, respondents gave answers that were compatible with the model to about two thirds of the proposed statements. This is better than half – the proportion that would be expected by chance – but far from a general consensus. This suggests that invoking the Swiss cheese model will not necessarily lead to effective communication, even among quality and safety professionals.
There was substantial variability among respondents as to what the various features of the model represent. The murkiest notion appeared to be the representation of the medical error itself. Few of the respondents recognised that an active error is a type of weakness in defences against patient harm within the health care system, represented by a hole in the Swiss cheese model (a "hole" is either an active or a latent error). The model is almost too successful in placing emphasis on systemic causes of patient harm, as opposed to an individual's failure.
The variability in interpretations revealed in this survey is more understandable if one considers the evolution of Reason's model between 1990 and 2000. In the first rendition of the model (Figure ), what was predicted was an accident, latent errors were placed as antecedents of the accident trajectory at the far left, and unsafe acts (i.e., active errors) were represented by a separate "slice" [2
]. Two subsequent models place the set of barriers between harm and the patient (the "slices of Swiss cheese") in a more global context (Figure and ). In particular, these models attempt to show causal chains that lead up to patient harm. For the sake of clarity, it should be noted that these more complete models have not been dubbed "Swiss cheese models." The model of 1995 (Figure ) shows a sequence of conditions and events leading to an accident, and defences and barriers are represented as intervening only after the occurrence of an error or a violation [3
]. The model published in 1997 (Figure ) depicts the Swiss cheese model as leading to human losses, not accidents [4
]. This supports the view that patient safety interventions should focus on patient harm, rather than errors [8
]. Importantly, the 1997 model also displays unsafe acts and workplace factors as orthogonal to the arrow leading from hazards to losses – presumably, each weakness in the system has its own set of causal or contributing factors. The current version of the Swiss cheese model (Figure ), published in 2000, appears to be a simplification of the previous model, from which the causal pathways have been removed.
Reason's model published in 1990 (2).
Reason's model published in 1995 (3), as adapted by Vincent et al (10).
Reason's model published in 1997 (4).
While no model ever claims to represent fairly a complex reality, it is possible that the latest rendition of the Swiss cheese model has become too simplified to remain effective in promoting patient safety. More realistic alternatives include the model of 1995 (Figure ), which clearly separates the event, or accident, from patient harm, and has remained in use [10
], and that of 1997 (Figure ), which suggests that the occurrence of a system failure cannot be easily represented by a simple linear sequence. In this Reason's model rejoins Haddon's matrix, a successful epidemiologic model for investigating injuries [11
]. The relevance of Haddon's matrix for investigating medical mishaps has been recognised by others [8
]. The integration of Reason's and Haddon's models may be a worthwhile next step toward a comprehensive model of patient safety.
More generally, the diversity of views documented in this study raises questions about the current status of a "culture of safety" among quality and safety professionals. A culture is a set of values, concepts and beliefs that are shared by a social group [12
]. The Swiss cheese model is the leading candidate for a common understanding of how harmful events occur and how they can be prevented. Until most or all actors agree on what the model means, the emergence and dissemination of a shared culture of safety may prove difficult. The danger is that people today use the label "Swiss cheese model" without realising that its meaning varies from one person to the next.
This study has several limitations. The main concern is that the sample of respondents was self-selected, and may not represent fairly the broader community of patient safety and healthcare quality professionals. It is likely that those who were most interested and most knowledgeable about patient safety are over-represented among participants. Secondly, it is possible that respondents misrepresented their level of familiarity with the model, and that results would have been better among true experts. Nevertheless, the discrepancy between self-perceived familiarity with the model and variable interpretations of the model features is striking. Finally, the questionnaire was developed ad hoc, and its reliability and validity are untested.
In summary, this study has shown that quality and safety professionals vary considerably in their interpretation of various components of the Swiss cheese model applied to medical mishaps. This finding echoes the variability in interpretation that exists even for basic terms of patient safety, such as "incident," "error," "mishap," etc. [14
]. Recent proposals of a comprehensive taxonomy of patient safety illustrate the necessity of a global conceptual model [15
]. Good models and clear concepts are required for a common terminology, and a common terminology is a pre-requisite for effective communication and progress in the field of patient safety.