The Heinrich/Bird safety pyramid
Pioneering research has become a safety myth
Herbert W. Heinrich was a pioneering occupational safety researcher, whose 1931 publication Industrial Accident Prevention: A Scientific Approach [Heinrich 1931] was based on the analysis of accident data collected by his employer, a large insurance company. This work, which continued for more than thirty years, identified causal factors of industrial accidents including “unsafe acts of people” and “unsafe mechanical or physical conditions”. Heinrich also put forward the domino model of accident causation, a simple linear accident model.
The work was pursued and disseminated in the 1970s by Frank E. Bird, who worked for the Insurance Company of North America. F. Bird analyzed more than 1.7 million accidents reported by 297 cooperating companies. These companies represented 21 different industrial groups, employing 1.7 million employees who worked over 3 billion hours during the exposure period analyzed.
The most famous result is the incident/accident pyramid, also known as the “safety pyramid” or the “accident triangle”. The pyramid, as illustrated by Heinrich in his 1931 book, is shown below.
F. Bird’s later work revealed the following ratios in the accidents reported to the insurance company:
For every reported major injury (resulting in a fatality, disability, lost time or medical treatment), there were 9.8 reported minor injuries (requiring only first aid). For the 95 companies that further analyzed major injuries in their reporting, the ratio was one lost time injury per 15 medical treatment injuries.
This work suggested that the ratio between fatal accidents, accidents, injuries and minor incidents (often reported as 1-10-30-600, and sometimes called “Heinrich’s Law” or the “Heinrich ratio”) is relatively constant, over time and across companies. Note that these numbers refer to accidents that were reported to the insurance company and incidents discussed with the researchers, which may be rather different from the real number of accidents and incidents. Indeed, more recent research suggests that these ratios are likely to be misleading:
It is impossible to conceive of incident data being gathered through the usual reporting methods in 1926 in which 10 out of 11 accidents could be no-injury cases.
Other empirical studies. Analysis of occupational accidents in the Netherlands [Bellamy et al. 2008] suggests that the “shape” of the incident/accident pyramids is dependent on the type of activity and risk type. Analysis of an industry-wide database of occupational accidents in Chile over a 28-month period [Marshall, Hirmas, and Singer 2018] finds that the ratio of fatal, serious and minor incidents varies depending on the base incident frequency, indicating that (even if the variation is small) the stable “Heinrich ratio” hypothesis is statistically invalid in this country and industry sector. A study of incident data in US mines attempted to test the predictive validity of the incident/accident pyramid by checking whether the number of incidents at a mine predicted the number of fatalities at the mine the following year [Yorio and Moore 2018]. The authors conclude that there is “no significant difference in the probability of a subsequent year fatal event between mining establishment that had a total of 16 or less total lost/restricted days and those that had none” (though mines with larger numbers of lost days did have larger probabilities of subsequent fatalities) and later that “the safety triangle is not as obvious and straightforward as many assume it to be”. Empirical studies in the medical area (emergency department attendance, medication errors) [Gallivan et al. 2008] find little evidence for a stable ratio between minor, intermediate and high severity events.
A counter-hypothesis: deviations as learning opportunities
Some studies even indicate that there can be, in certain sectors, a negative correlation between the number of recorded incidents and fatality rates (companies where more incidents are reported have fewer deaths). For example, a study of nonfatal accident/incident rates and passenger mortality risk in US airlines in the 1990s found a negative correlation between the number of non-fatal incidents and accidents recorded by airline companies and the probability of a passenger dying on one of their flights [Barnett and Wang 2000]. A study in the construction industry in Finland [Saloniemi and Oksanen 1998] found a negative correlation between the total number of recorded accidents and the number of fatal accidents. There are two possible explanations for these observations:
A low rate of recorded incidents and accidents may be caused by underreporting, where incidents are hidden by frontline staff or their managers instead of being reported. Multiple factors can contribute to underreporting, including a blame culture, lack of resources for experience feedback and corporate “zero accident” targets. Underreporting means there is little learning from the “gift of failure” and hinders safety improvements, so a company with few reported incidents may, in fact, be less safe than one where incidents are reported and lessons are learned.
Non-fatal incidents and accidents provide opportunities for operational staff to understand how their system reacts in abnormal conditions, and provides a form of “training” in managing deviations and bringing the system back to control. They help prevent workers from becoming complacent about the risks in the workplace. (Naturally, it would be safer if this experience could be accumulated on a realistic simulator, during controlled training periods, but such training is expensive.)
These two possible explanations are non-contradictory.
Disputed findings on accident causality
Heinrich’s work was pioneering in analyzing the causal factors that led to workplace accidents, highlighting the associated costs and encouraging managers to think about and invest in prevention of occupational accidents (interrupting an accident sequence). However, some of these findings on causality were affected by biases.
One conclusion of Heinrich’s work is that 95% of workplace accidents are caused by “unsafe acts”. Heinrich came to this conclusion after reviewing thousands of accident reports completed by supervisors, and interviewing these supervisors as much as ten years after the relevant incidents. These supervisors are likely to often have blamed workers for causing accidents without conducting detailed investigations into the root causes, which would probably have revealed other causal factors such as unsafe machinery, management pressure to work quickly, and poor information on hazards. These other causal factors are the responsibility of managers, and it is well known that people have natural psychological tendencies to downplay their own contribution to negative outcomes and attribute them instead to other factors (in this case, the “sharp end” workers/victims).
Another disputed finding in Heinrich and Bird’s work concerns the causality of minor incidents and of major accidents. Heinrich stated that:
predominant causes of no-injury accidents are, in average cases, identical with the predominant causes of major injuries, and incidentally of minor injuries as well.
This is incorrect in high-hazard industry today, and can lead to inappropriate allocation of resources. In particular, it leads some companies to an excessive focus on “behavioural safety”, workplace housekeeping and the prevention of low-consequence incidents such as slips and falls, to the detriment of investment in maintenance and technical and organizational safety improvements. Contrary to Heinrich’s assertion above, in industries concerned by major accident hazards, there are significant differences between major accidents and minor incidents: these differences include the activities involved, the amounts of energy released, the characteristics and the numbers of safety barriers that were or could have been relevant to the event.
Accident causality is often more complicated than Heinrich’s quote suggests, as indicated by the following extract from the BP report into the Deepwater Horizon accident:
The team did not identify any single action or inaction that caused this incident. Rather, a complex and interlinked series of mechanical failures, human judgments, engineering design, operational implementation and team interfaces came together to allow the initiation and escalation of the accident.
Major accidents in high-hazard industries are generally caused by some unlikely combination of circumstances that was not controlled due to poor decision-making, management pressure for performance at the expense of safety, poor communication, or unexpected interactions between different components of complex systems. These causal factors are very different from “unsafe acts”, and their management requires specific actions by safety specialists, system designers and system managers that is unrelated to behavioural safety.
Even Heinrich himself felt it necessary to warn against incorrect interpretation of his work, and stated in the fifth edition of his book, published in 1980 ([Heinrich, Petersen, and Roos 1980], cited in [Hale 2002]):
There has been much confusion about the original ratio in industrial accident prevention. It does not mean, as we have too often interpreted it to mean, that the causes of frequency are the same as the causes of severe injury. National figures show that different things cause severe injuries than the things that cause minor injuries. Statistics show that we have been only partially successful in reducìng severity by attacking frequency.
In fact, the rate of minor events (measured by personal injury rates, with indicators such as TRIR) is often mistakenly used as a proxy for overall safety performance, and can lead top management to an incorrect view of the safety of an activity. This occurred at BP’s refining activities and contributed to the Texas City accident in 2005. Indeed, as the 2007 Baker report on the explosion indicates:
BP mistakenly interpreted improving personal injury rates as an indication of acceptable process safety performance at its US refineries. BP’s reliance on this data, combined with an inadequate process safety understanding, created a false sense of confidence that BP was properly addressing process safety risks.
Similarly, Andrew Hopkins writes [Hopkins 2001] concerning the 1998 gas explosion at an Esso plant in Longford, Victoria, Australia:
Ironically Esso’s safety performance at the time, as measured by its Lost Time injury Frequency Rate, was enviable. The previous year, 1997, had passed without a single lost time injury and Esso Australia had won an industry award for this performance. It had completed five million work hours without a lost time injury to either an employee or contractor. […] LTI data are thus a measure of how well a company is managing the minor hazards which result in routine injuries; they tell us nothing about how well major hazards are being managed. Moreover, firms normally attend to what is being measured, at the expense of what is not. Thus a focus on LTIs can lead companies to become complacent about their management of major hazards. This is exactly what seems to have happened at Esso.
The work of Heinrich and Bird, and the “safety pyramid” model, are widely used in safety training to justify a focus on behavioural safety (reducing the occurrence of unsafe acts, wearing individual protective equipment, following work procedures strictly, increasing attention to identify workplace hazards). It is a useful mental image that helps highlight that
- major injuries are rare events
- more frequent, less serious events provide opportunities to improve safety
The pyramid metaphor is sometimes combined with that of an iceberg, where the visible part above the waterline consists of reported injuries and fatalities, and the invisible part under water are all the unreported incidents and near misses. This mental image helps emphasize that there is potential for safety improvement in incidents that do not always get registered in the official reporting system, so it’s worthwhile trying to increase the visibility of these events.
While this mental image is positive in helping prevent occupational accidents (and is clearly very “sticky” in people’s minds…), it is often misinterpreted in ways that reduce attention paid to major accident hazards. One common misinterpretation is “frequency reduction will trigger a severity reduction”. This is a “structuralist” view of the Bird pyramid, a mistaken view or myth1 that “chipping away at the minor incidents forming the base of the pyramid will necessarily prevent large accidents” [Hale 2002]. It assumes that there is a common cause between minor incidents and major accidents (“what hurts workers is also what kills them”), which is only partly true, as discussed above. It suggests an intervention strategy that is fairly easy to implement (if paternalistic): “focus people’s attention on avoiding minor incidents (slips, trips and falls) and their increased awareness of minor safety problems will prevent the occurrence of major events”. This interpretation is false concerning process safety and major accident hazards, which require specific focus, as mentioned above.
We have seen that the descriptive validity of the Heinrich/Bird incident/accident pyramid is lower than many safety professionals believe. More importantly, its predictive validity concerning major accident hazards is very low, because the factors that cause low-severity incidents are typically quite different from the factors that cause high-severity accidents. It is time to put this safety myth to rest.
Barnett, Arnold, and Alexander Wang. 2000. “Passenger mortality risk estimates provide perspectives about flight safety.” Flight Safety Digest 19 (4): 1–12. https://flightsafety.org/fsd/fsd_apr00.pdf.
Bellamy, Linda J., Ben J. M. Ale, J. Y. Whiston, M.L. Mud, H. Baksteen, Andrew R. Hale, I.A. Papazoglou, A. Bloemhoff, M. Damen, and J. I.H. Oh. 2008. “The software tool Storybuilder and the analysis of the horrible stories of occupational accidents.” Safety Science 46 (2): 186–97.
Hale, Andrew R. 2002. “Conditions of occurrence of major and minor accidents: Urban myths, deviations and accident scenarios.” Tijdschrift Voor Toegepaste Arbowetenschap 15 (3). Delft. http://www.arbeidshygiene.nl/-uploads/files/insite/2002-03-hale-full-paper-trf.pdf.
Heinrich, Herbert William. 1931. Industrial accident prevention: A scientific approach. New York. McGraw-Hill.
Hopkins, Andrew. 2001. “Lessons from Esso’s gas plant explosion at Longford.” In Proceedings of the First National Conference on Occupational Health & Safety Management Systems, edited by Warwick Pearse, Clare Gallagher, and Liz Bluff, 41–52.
Safety researcher Andrew Hale refers to “beliefs which seem so plausible that they command immediate acceptance”.↩