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Vishwadeep Khatri

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Heinrich's Accident Triangle is an industrial accident theory proposed by HW Heinrich in 1931. It is based on an assumption that the frequency of incidents is inversely proportional to the severity of the incident. Heinrich's assumed numbers are - for every incident that causes a major injury, there are 29 incidents that cause minor injuries and 300 incidents that cause no injuries.


An application-oriented question on the topic along with responses can be seen below. The best answer was provided by Mohammad Mahmudul Hassan on 1st Oct 2021.


Applause for all the respondents - Meenakshi Iyer, Varuna Kakathkar, Mokshesh, Saravanan MR, Satinder Singh, Johanan Collins, Mohammad Mahmudul Hassan.


Q 405. Explain Heinrich's Accident Triangle that helped shape the safety and security in organizations in the last century. What are some of the limitations of this approach for workplace safety and how can they be overcome?


Note for website visitors - Two questions are asked every week on this platform. One on Tuesday and the other on Friday.

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Heinrich’s Accident Triangle


In the theory of industrial accident prevention, the accident triangle is known as Heinrich triangle or Birds triangle. The triangle first introduced by H.W Heinrich in 1931 and later expanded by Frank E. Bird (1969), It shows the relationships between Major, Minor and near miss. The triangle proposes that accidents of minor accidents are reduced then the corresponding fall of Major accidents. The triangle is regarded as a corner stone of 20th century work place of health and safety of philosophy. Heinrich’s Law has significant impact on the industrial safely long before OHSA created.



Fig: Heinrich’s 300-29-1 Model and Bird Model


In 1931, H.W Heinrich researched on 75000 no’s work injuries when he worked in an insurance company and wanted to reduce serious/major accidents.  He surprised and notices that similar injuries pattern occurs so regularly.  Then he created Heinrich’s Pyramid also called Heinrich’s Triangle or Heinrich’s Law.  He observed that every 300 near missed event causes 29 minor injuries and one major injury or fatality.  Henry believes that to eliminate injuries, first needed to eliminate the minor incidents.


Fig: The top of the pyramid is most difficult to shift according to Heinrich’s Model


Heinrich’s thoughts if bottom of the pyramids is eliminated will also eliminated the top of the pyramids.  This theory is called the Heinrich’s Law.  This theory is widely used and dissed today. However, from lesson learned from the Heinricks that that it helps to reduce the minor injuries overall but it does not but it does not eliminate the serious risk , injuries and fatalities at all.  Its turn out that the top of the pyramid is most difficult to shift that Henricks found,




It’s truly reduced the most serious incidents, also had to be focuses on Low frequency and high severity of consequences. For example, falls from height, Mobile tower accidents, Fires and explosions. In his experiment illustrates that truly reduces the near misses and minor injuries. But only tells part of the stories. To reduce the serious injury and fatality need to focuses on the low frequency and high severity events.  In these approach together removes all injuries and fatalities.


Heinrich believed and his research presented that majority of accidents (88%) were a result of unsafe acts of workers or caused by human decision. 10 % to unsafe conditions, and rest 2% as unpreventable.  He suggested that and individual’s life experiences and background could influence them to take risks during work accomplishment.  He believed that removing a single casual factor could result in preventing an accident.  


In 1966, Frank E Bird analysis on 1.7 million accidents reports from around 300 companies and He amended the triangle showing relationship of one major injury accident caused by 10 minor injury caused by 30 property damage accidents causing accidents that results to 600 near misses.  He claimed that majority of accidents could be predicted and prevented by appropriate innovations.


Limitations and Challenges of Heinrich’s Model :

The Henrich model was unchallenged for many years. However, some recent studies Henrichs equilateral triangle shape was challenged. Some professional believe that the actual shape would depend on the organizational structure and culture.  In 2010 report of a large gas and oil company, the original values of the Henrich’s are true only applied to large datasets and broad range of activities.




Fig:  Most experts and scientist are told parameters are not scientifically accurate today.   


Recent Research and study about limitations:

In a study in 1991, in confined spaces the triangle relations were significantly different, 1.2 injuries for each serious major accidents or fatality. In UK mid 1990’s shows the triangular relationships 1 fatality to 207 major injuries, to 1402 injuries causing to three or more days lost in time injuries, to 2754 minor injuries.  Henrich original file was lost since the accident figures could not be proven.


W Edward Deming criticism about the Henrich’s Model:

He criticizes the human actions in work places of most accidents (88%) which is incorrect to him, in fact poor management system causes the majority of the accidents. There is also criticism on the focusing on reducing minor accidents and claim that work place supervisor to ignore more severity and low frequency risk when planning works.


How we can overcome the limitations:

The industries are becoming changes and digitalized. Lot of IOT, POKA YOKE devices and sensing devices are used for safe working environment. Robots are increasingly used in in the hazardous activities. Digital Twin are the breakthrough technologies that can be used for industry sector. 





At the end, but an almost century later and in the changed and technological industrial environment and the safety sense has also changed, even today plenty of research relying that by reducing the number of near miss will decrease the major accidents which is based on Heinrich’s Principle.


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William Herbert Heinrich during 1920-30 saw ~75000 industrial incidents, workplace injury and illness case. He concluded that every 300 near miss has 29 minor and 1 major injury, to eliminate the major injury reduce the near miss cases. Heinrich triangle is pictorial representation of relationship between near miss incident and accidents. This is used to identify where the risk are high and where to investigate to prevent risk

Heinrich Theory:

1)      Behind every accident there is a cause

2)      Accident don’t happen but they are caused

3)      78% of accidents cause due to unsafe act

4)      20% of the accidents are due to unsafe condition



Top of the pyramid is more difficult to shift. To reduce incidents, we need to focus on high frequency and severity event

The incidents are caused due to 3 broad reasons

a)       Technical glitch b) Human Performance c) Missing barrier


Henrich’s triangle was criticized in recent period

a)       Due to the # incidents used in the Pyramid stating the value applicable to large datasets, however in limited set the relationship is not 1:29:300 but in 1991 it was 1:1.2 (for 1.2 minor incident, there is 1 serious incident). In 1990 the relationship was indicated as 1:207:1402 and many more such studies leading to limiting the accidental # provided in Henrich’s triangle.

b)      The focus of human error is to be shifted to poor management.

c)       The focus on reducing minor incident would lead to ignore serious incidents






Henrich’s triangle had following limitations:

1)      Statistics cannot be validated: The ratio of Near miss with major incident is not as per recent data availability.

2)      Data issue (Skewness/inaccuracy etc.): The dataset might not consider all the available scenario or can be skewed leading to inaccurate ratio.

3)      No assumption/potential: The pyramid considered actual cases and not look at the potential risk

To overcome the limitations following points can be considered

a)       Replace traditional Pyramid with latest scenario, data, and sequence

b)      Collect more data and build model indicating warnings for potential cases also

c)       Conduct behavioral observation program

d)      Set up Leading indicator to provide ratio of action item to number of incidents, Behavior to reduction of incidents, Near miss to injury


Bird, F.E., & Germain, G.L. (1996). Practical loss

control leadership (Revised edition). Loganville, GA: Det

Norske Veritas.

Collins, R. (2011, June). Heinrich’s fourth dimension.

Difford, P.A. (2011). Redressing the balance: A

commonsense approach to causation. Bridgewater, U.K.:

Accidental Books Ltd


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What is Heinrich's Accident Triangle?


The accident triangle was developed by H.W. Heinrich, an American industrial safety pioneer in 1931. He was an assistant superintendent working with an insurance company when he worked on his theory on behaviour based safety. His work was an important foundation to the safety philosophy in 20th century. He proposed that there is a numerical relationship between near misses, minor injuries, and major (or fatal) injuries at a work place. His theory was further updated and expanded by Frank Bird in late 1960s and hence also called as Bird’s triangle.


In their research, both Heinrich and Bird stated that, if the number of minor accidents (or near misses) is reduced then there will be a corresponding fall in the number of major accidents. Heinrich did an extensive study on 75000 accident cases from insurance company’s database and industrial safety records. Through his analysis he proposed a relationship as below:


300 minor injuries/accidents -> 29 minor injuries/accidents -> 1 major injury/accident


He drew the conclusion that, there exists a “domino model” of events that works on principle of “linear causal model” with a mathematical relationship as shown above. He further stated that, by reducing the number of minor accidents, industrial companies would see a correlating fall in the number of major accidents. The relationship is often depicted in the form of a triangle (shown below) and hence called as Heinrich’s Accident Triangle.



How Heinreich’s Accident Triangle helped in industrial safety?


Heinrich’s study showed that:


1. A lower-severity event can be used to predict a future fatal event within the same work place. Or in a simpler way, if a work place has enough near misses then the same workplace will eventually face a serious injury.

2. 95% of work place accidents were due to unsafe work practices and behaviours of workers and management.

3. Many accidents share common root causes (often near miss accidents). Hence addressing more minor accidents that cause no injuries can prevent accidents that can cause major injuries.

4. When employer focuses on what workers do, analyses why they do it, and then applies a research-supported intervention strategy to improve what people do will create a safety partnership between management and employees to prevent major accidents/injuries. This partnership is called as behaviour based safety (BBS) approach.

5. BBS program must include all employees, from the CEO to the front line workers as changes cannot be done without buy-in and support from all involved in safety related decision making. At the core, BBS is based on organizational behaviour management.


Limitations of Heinrich’s Accident Triangle


Heinrich’s ideas were considered sacred, until Fred Manuele, challenged the validity of Heinrich’s Law. Manuele through his theory claimed that if small incidents are managed effectively, the rate of occurrence of small incident declines, but the probability of major accident rate stays the same, or even slightly increases.

Many more experts including Deming further criticized Heinrich’s theory owing to following reasons:


1. Ratios postulated by Heinrich (1 : 29 : 300) do not work when applied to specific activities and sectors. The method used to develop said mathematical relationship itself is unclear and being questioned.

2. Classification of incidents into major and minor by Heinrich could be skewed by the fact that not all minor incidents have outcome of same gravity. Due to this skewness, a minor incident may get classified as near miss or vice versa.

3. Underreporting of minor incidents or near misses by front line staff or their managers can cause skewness in reporting. This can significantly weaken the approach to implement Heinrich’s triangle approach.

4. Heinrich’s theory when practiced, led to excessive focus on the minor accident prevention, at the cost of losing sight of potentially major accident causing activities.

5. Heinrich’s triangle has an excessively simple approach of behaviours leading to incidents/accidents. It excessively focuses on training and procedural compliance.

6. Heinrich’s belief that behavioural aspects leads to mistakes tends to put up excessive blame on individual or group of people for any failure incident.

7. Heinrich’s theory adopts a linear model of causality which is inappropriate to explain the failure of modern complex systems. Complex systems fail in complex ways.

8. Heinrich’s model focuses on actual accident outcomes of various magnitudes and lacks focus on risk potential of various magnitudes.


Methods to Overcome Limitations of Heinrich’s Model


1. Near misses and minor incidents must be assessed in light of injury and fatality potential. For example, a near miss of worker from falling in an empty reactor vessel v/s falling into a tank of acid storage must be assessed in light of injury and fatality potential.

2. Focus should be built on eliminating incidents with high potential of injury and fatality.

3. Approach to consider behavioural mistake at the core of every incident must be changed. Thinking that a flawed process or a poor system design can cause incidents must be instilled.

4. Approach of process re-engineering and system/equipment re-design to avoid incidents must be adopted instead of constantly working on improving human behaviours. Shifting from Baka Yoke (fool proofing) to Poka Yoke (mistake proofing) ideology.

5. Adopt complex causal analysis using tools like “Why Because Analysis/Graph”, “Event Tree/Fault Tree Analysis“, FMEA, etc.. to understand the failure of complex systems instead of relying on simple linear causal model (domino effect) to address the potential risks.







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Heinrich's Accident Triangle is also known as Bird's Triangle or Accident Triangle. It shows the relationship between serious and minor accidents and also near misses. This theory proposes that If the number of minor accidents get's reduced, then the number of severe accidents also will get reduced proportionately. And hence there is a need to reduce the minor accidents. The theory was first proposed by William Heinrich in 1931.




Post that in 1966,this theory was further developed by Frank E Bird based on the data obtained from 1.7 million reports obtained from ~ 300 Companies. He showed a relationship of 1 fatal incident to 10 serious accidents to 30 minor accidents to 600 near misses. But it should be noted that here only the reported cases are taken into account and not the unreported one.


The major use of the triangle is to convey the message that Serious accidents tend to happen rarely whereas on the other side near misses/incidents would happen more frequently but usually having less consequence on the serious injury. But if they keep un attended, may result in some serious injury. Hence these near miss incidents should be thoroughly investigated with the preventive measures to avoid the reoccurrence.


This triangle has limitations as well.


1)Reliability of data: Here the Triangle assumes that the data provided by an Organization is correct and nothing has been missed. But in practical scenario, there are many unreported cases as well.

2)Data sets may be skewed: Not all the unsafe acts and near misses are of same intensity. There may be certain accident and rather than reporting as near miss, if it had been mentioned as minor accident then the comparison may not be the right.

3)Results are based on the actual outcome of the event: The possibility of certain major accident need not be reported as a higher volume of minor accidents earlier.


Even though it is not possible to completely eliminate the limitations can be reduced by

1).Ensuring all cases have been entered without any miss.

2).By providing some predefined self weightage to make the intensity of unsafe and nearby cases more linear.





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Heinrich accident Triangle:

In 1930, Heinrich was working in travel insurance and published the theory called Heinrich triangle after his extensive study about health and safety in workplace accidents. This theory became popular and also called Heinrich’s law.

The Heinrich law triangle shows the relationship ratio with the number of accidents / incidents resulting in no injury (near miss / unsafe act / condition), minor injury (First aid / NLTA) and serious injury (LTA / Fatality), The ratio is 300-29-1.

He also emphasized that 88% of accidents are due to human decision towards unsafe acts. He has concluded this after studying the report generated by supervisors. Hence, One of the criticisms about the Heinrich triangle is that the model over emphasizes the people's behavior and not much attention was given to systems and procedures.

It is based on probability and assuming that the number of accidents is indirectly proportional to severity of the accident. The conclusion is that a minimum number of minor incidents will reduce major accidents, But that is not necessarily the case.



The above pictures show that 3000 unsafe acts / conditions, 300 near misses, 29 minor injuries lead to one fatal accident. Most of the organizations conduct root causes analysis on some critical injury and fatal accident which is more visible in nature but no one works on near miss, unsafe condition and acts. 


The below pictures shall provide the meaning of unsafe act / condition, near miss and accidents.  However the definitions are provided below for better understanding.




Unsafe condition and acts:

Hazardous or physical conditions or circumstances which lead to an accident are called unsafe conditions whereas an unsafe act is a violation of procedure or system which could lead to accidents.


Near miss:

Safety incident in which no injury and/or illness happens, but has the potential to do so under slightly different conditions.



Potentially serious safety event: The  near miss or unsafe condition / act which by  associating credible aggravating factors  with the actual conditions, could have led to a major or serious safety incident or an occupational disease or a fatality.


Examples of aggravating factors:

The human or organizational factor, 

Equipment failure, 

an external factor (EX. adverse weather conditions,)




LTA Lost time accident:

 Injury to and/or work-related illness of an employee be absent from the workplace for a minimum of one full workday.


NLTA Non-lost time accident:

Injury to an employee, while working does not prevent him/her from returning to work on the same day, or the day following the accident, after a medical treatment or upon such prescription by a licensed healthcare professional.

NLTAs are divided into:

  • NLTA without restriction: when the impacted person can normally resume his/her regular duties,

  • NLTA with restriction (also called modified duty): when the impacted person can resume his/her regular duties but with a temporary restricted work assignment, light duty or reduced working hours.

First aid: 

Injury to and/or work-related illness of an employee while working, that:

  • does not prevent the person from resuming his/her work on the same day, or the day following the incident.

The following examples are considered as first aid:

  • using non prescription medication,

  • cleaning, flushing or soaking superficial wounds,

  • using wound coverings such as bandages, steri-strips™, 

  • using temporary immobilization,


Death, through injury and/or illness, of one person, while working.



The ultimate aim of the Heinrich triangle is to widen the bottom of the triangle to identify and analyze the at risk behaviors / unsafe conditions / unsafe acts / near misses to avoid any major injuries. These are all called leading indicators, Here the important thing is all the observations like unsafe act / condition / behavior / near misses shall be analyzed through root cause analysis / 5 why / HAZOP study / 8 D method or any other problem solving methodology to identify the correct root cause and terminate it permanently. Nowadays most of the companies are working more on leading indicators rather than working on lagging indicators. If the organization achieves the target of leading indicators then it will eliminate any accident or incident. It means we can easily achieve our lagging indicators ( 0 LTA / 0 NLTA / 0 Fatal accidents) by identifying and analyzing all the leading indicators. BBS (behavior based safety also one of the leading indicators to engage people and have dialogue with them to understand the real problems. The below picture shows the same about leading and lagging indicators. 


Most of the organizations work on tip of the iceberg means on the lagging indicators after the accident occurred but matured organization work on the invisible part of the iceberg where leading indicators are appearing. Somehow, The same concept is applicable for CoPQ (cost of poor quality), we always work on the major customer complaints and forget to take permanent action of local NCR non conforming report.




We have to work on invisible area to strengthen the safety and quality system



Thanks for reading :)


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Heinrich accident triangle is a theory of Industrial accident prevention, which is also known as Bird’s triangle. It shows the relationship between the three types of accidents.

1.  Serious accidents

2. Minor accidents

3. Near misses

It is generally depicted as a pyramid where serious accidents comes at the top (tip of the pyramid), minor accidents in the middle (middle part of the pyramid) and near misses comes at the bottom (base of the pyramid).


Heinrich’s theory is based on the probability and assumption that number of accidents are inversely proportional to the severity of those accidents and if we are able to reduce the number of near misses and minor accidents the number of serious accidents will automatically reduce.


There are few limitations of Heinrich accidents which are as follow:

1. It blames too much on worker: This theory blames too much on the individual without considering the root cause of the accident. Which encourages a culture where individual not even report the problem.


2. It focuses too much on reducing minor accidents: This theory over states the relationship between minor accidents and serious accidents which intern lead to a problem like ignoring more serious but less likely accidents.


These Limitations can be overcome by

1. Focusing on finding out the root causes of the problem. Introducing better management systems that can eliminate the root causes of accidents. For instance, using safety systems like LOTO (Lockout tagout), Confined space entry, machine guards, PTW (Permit to Work)

2. Focusing on more serious but less likely risks also, as they can also cause serious accidents 

3. Corrective actions must be taken to manage ongoing risks.

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Heinrich's triangle

Heinrich Triangle or Bird Triangle is also called the Accident Triangle. It is used in the prevention of industrial accidents and shows the relationships between near misses at the bottom of the triangle, minor accidents in the middle, and more serious/fatal accidents at the top of the triangle. The theory is that if the number of near misses is reduced, the proportional minor accidents and serious/fatal accidents will reduce. In the Swiss Cheese analogy, the increase in the number of slices will lead to the reduction of direct holes passing right through the block of cheese leading to a reduction in the number of accidents.

This theory was first proposed by Herbert William Heinrich (1931).  He studied about 75,000 accident reports that occurred at the industrial sites and from the insurance reports. The theory was fine-tuned by Frank E Bird by studying about 1.7 million industrial accidents. This is about 22 times the accidents studied by Heinrich. The resultant Accident Triangle from the study is widely used in Industrial Health and Safety programs.

Who was Heinrich?

Heinrich was an Assistant Superintendent working in an insurance company. His study was aimed at reducing the number of industrial accidents. He found that the ratio of the major injury accidents: minor injury accidents: no-injury accidents was 1:29:300. His logical conclusion was that the reduction of minor accidents will naturally lead to a reduction of major accidents. This ratio is depicted as the Heinrich Triangle.

Heinrich’s Triangle in our Daily Life.

Heinrich’s Triangle finds natural and logical use in our daily life. If we take concrete steps to make our living environment at home and office at office safe, it would naturally lead to the probability of major accidents reducing. For example, ensuring that the maintenance schedules of your car or other mechanical devices are carried out as per schedule, will lead to a reduction in accidents. One can say that following Heinrich’s Triangle in our personal lives would lead to a culture of safety in our near proximity.



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