Is the Principle of a Stable Heinrich Ratio a Myth?
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Background: Safety improvements are sometimes based on the premise that introducing measures to combat minor or no-harm incidents proportionately reduces the incidence of major incidents involving harm. This is in line with the principle of the Heinrich ratio, which asserts that there is a relatively fixed ratio between the incidence of no-harm incidents, minor incidents and major incidents. This principle has been advocated as a means of targeting and evaluating new safety initiatives.
Research Methodology: Both thought experimentation and analysis of empirical data were used to examine the plausibility of this principle. A descriptive statistical analysis was carried out using triangle plots to display the relative frequencies of the occurrence of safety incidents classified as minor, moderate or severe.
Findings: Thought experiments indicated that the principle of a fixed Heinrich ratio has a dubious logical foundation. Analysis of emergency department attendance and studies of medication errors demonstrated marked variation in the relative ratios of different outcomes. Triangle plots of UK road traffic accident data revealed a hitherto unrecognized systematic pattern of change that contradicts the principle of the Heinrich ratio.
Interpretation: This study of the principle of a fixed Heinrich ratio invalidates it: introducing measures to reduce the incidence of minor incidents will not inevitably reduce the incidence of major incidents pro rata. Any safety policies based on the assumption that the Heinrich ratio is true need to be rethought.
KeywordsMedication Error Equilateral Triangle Road Traffic Accident Major Incident Industrial Accident
This work was funded by the Department of Health as part of its Patient Safety Research Programme. The study sponsor did not have any role in the study design; collection, analysis and interpretation of data; or the writing of the paper. Contributions of authors: All authors were involved in designing the study and writing the final paper. Katja Taxis and Bryony Dean Franklin did most of the literature search; Steve Gallivan wrote the vignettes, developed the graphical method and did the data analysis; Nick Barber is the overall guarantor of the study. None of the authors have any conflicts of interest that are directly relevant to the content of this article.
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