Is the Principle of a Stable Heinrich Ratio a Myth?

Abstract

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. 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. 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. 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.