Session No. 534

Using Accident Theories to Prevent Accidents

Jeffrey S. Oakley, Ph.D., CSP Safety Engineer

The Boeing Company – NASA Systems Houston, Texas

Introduction Why should you wait until there is an accident at your workplace before you perform safety

measures? Why should you wait until there is an accident to use the accident theories? Accident

theories have been used for accident investigation and causal analysis for many years. It is time

to start using these techniques in a proactive approach. This paper will teach you how to use

these accident theories and analytical techniques used in accident investigation as proactive safety

techniques that you can use to identify, analyze, and prevent hazards from becoming accidents.

This paper will discuss accident theories, accident investigation techniques, system safety

techniques, and other techniques that can be used in a proactive manner to prevent accidents from

occurring in the first place. These techniques will be analyzed from an accident theory approach.

There are many different types of accident theories that contradict each other so many different

types of techniques and theories will be discussed.

This paper will address the one question safety engineers have pondered for centuries. How

do we prevent accidents? -- By first taking a look at how accidents occur. Accidents do not just

happen--they are caused and the key is to find the causes and control them before there is an

accident. This paper will look at many of the available accident theories and analyze how and

why accident occur and discuss how to use them as proactive tools to prevent accidents. The

purpose is to help use these techniques in the workplace to prevent future accidents by analyzing

what happened.

What Is An Accident? While many books will agree that an accident is an undesired event, the best definition that fits

with the accident theories and analytical approach is that occurrence in a sequence of events that

produces unintended injury, death or property damage (National Safety Council viii). Accidents

are sequences of events. There are normal (positive) sequences where there is no accident, and

then accident sequences also called negative sequences. An accident is a result of a negative

sequence of events. Figure 1 displays a simple way to look at accidents and the facts or events

that occurred.

FACT FACT FACT ACCIDENT FACT

TIME

Figure 1. The facts and events that lead to an accident (Oakley 66).

Analytical Approach to Accidents The analytical approach to investigating accidents is used to find out what happened and how to

prevent future accidents. This is a very intuitive and iterative process to use techniques that

develop scenarios and determines what happened. The purpose is to use techniques and

methodologies that help to determine the accident sequence and then prevent that sequence by

corrective actions or controls. In the analytical approach as illustrated in Figure 1, the purpose is

to use the techniques and methodologies to analyze the knowledge and facts to develop results or

recommendations and corrective actions to prevent accidents. An analytical approach will

establish consistency and validity to the proactive process.

Types of Accidents There are many types of accidents and even OSHA categorizes them based on severity. While

this is the regulatory process, the type of accident makes no difference. First aid injuries or

fatalities and catastrophes are basically all the same. The theories apply to both the small

accident and the large accident and even near misses. There is a sequence of events for all and

while some are more complicated than others, they are basically the same. Many of these

accidents are near misses because of luck more than safety controls. No matter what size of the

accident (even near misses), they all have causal factors that caused the accident. They key to

proactive safety is to identify the hazards and correct them before the accident.

How Accidents Occur? Accidents are much more than the old clichés – you were at the wrong place at the wrong time,

you were unlucky, Boy you were due for an accident. Accidents can get anybody at anytime,

however if one knows how they occur then there are steps to avoid them. Many employees put

up a strong façade – It won’t happen to me, I don’t need to work safe or wear this personal

protective equipment. This carelessness and lack of respect to safety is usually what starts an

accident sequence.

There are many accident models. If you understand how they occur, then you are better able

to prevent them from happening. These come in the form of medical theories, domino theories,

human factors (human error) theories, and theoretical models or science fiction “butterfly effect”

theories to name a few types. Table 1 lists these basic models that make up most of the accident

theories. The medical theories are best explained by the new television shows. Crime Scene

Investigation (CSI), medical shows, and the law shows use forensics to determine the crime or

accident by using medical knowledge to find the cause. Some of these shows even use numbers,

vision, and even special powers to figure out these accidents.

Accident Models

1. Medical Models

2. Domino Models

3. Human Factors/Human Error

4. Theoretical Models (Sci Fi)

Table 1. Accident Models make up many types.

Most safety engineers have used various domino theories to understand how accidents occur.

They start with a sequence of events that lead to other sequence of events, which eventually lead

to an accident. Just like a domino, if all fall down then it creates an accident. In order for this to

occur, all dominoes must be a cause or a negative path. If any of these dominoes were held up –

then there would be no accident.

There has been much debate about the human factors or human error theories. This model

relates the human part of the equation. How much the human or error relates to how an accident

occurs has been debated for years. While human behavior and actions are usually a factor in

many accidents, its causal relationship has been decreased in the safety and accident prevention

realm.

The last type of model is the theoretical concept or science fiction style or “butterfly effect”

phenomenon. How does a butterfly flapping its wings in France affect a person in the United

States? While this is a unique concept, the key to workplace is every action and condition can

affect people at the workplace. What the process operator at a chemical plant does can reflect on

another employee. This is the model that we use to tell our employees that we all need to work

safe, because our actions can impact our fellow employee.

Accident Theories There are many theories about why and how accidents occur, and understanding them is

important. These theories are continually challenged and revised, and some of the theories

contradict each other (Oakley 15). There is no real right answer as to which one of the accident

theories is correct; it all depends on the type of model that you use. Most of these depend on your

personal philosophy or your companies’ philosophy. The theories that will be discussed are listed

in Table 2.

Accident Theories

1. Accident Ratio Study

2. Domino Theories

3. Multiple Cause Theory

4. Epidemiological Theory

5. Haddon Matrix

6. Technical/Engineering Theories

7. Human Error/Human Factors Theories

8. Sequence of Events

Table 2. Accident Theories.

Accident Ratio Study While this is not necessarily an accident causation theory, it does develop a relationship between

major injuries, minor injuries, property damage, and near misses (close calls). Many of these

pyramid relationships have been developed with different numbers, but the same concept is

represented. Figure 2 illustrates the accident ratio study and Bird and Germain sums up this

concept as “ The 1-10-30-600 relationships in the ratio indicate quite clearly how foolish it is to

direct our major effort at the relatively few events resulting in serious or disabling injury when

there are so many significant opportunities that provide a much larger basis for more effective

control of total losses” (Bird and Germain 21). This ratio tries to get the safety engineer to use

more proactive approaches from near misses, close calls, and hazards to prevent the accident.

Figure 2. Accident Ratio Study.

Domino Theory There are many types of domino theories that have been developed over the years. The original

was Heinrich’s domino theory of accidents. “Heinrichs version of the domino theory illustrates

how an accident occurs by comparing the events leading up to it to a set of dominos. The first

domino (the first event) sets the stage and starts the accident sequence. When it falls, it pushes

the next, and that pushes the next, until the last domino, which represents the accident or injury, is

toppled” (Oakley 18). Figure 3 represents this domino theory. As a proactive approach,

“Heinrich showed that by removing one of the intervening dominos (a preventative action) the

remaining ones would not fall, and there would be no injury” (Ferry 127).

Figure 3. The Domino Theory starts with lack of control-basic causes-immediate causes-

incident-loss.

1 Major Injury

10 Minor Injuries

30 Property Damage

600 Near Misses

Another domino theory is the loss causation model, which starts with lack of control, basic

causes, immediate causes, incident, and then loss. This model defines that the control of the

situation, policy, supervision, or safety is lacking which started the domino and the negative

sequence. A personal or job factor that influences the negative path then starts the accident

sequence. The next step is waiting for the unsafe act or condition and then an incident (Bird and

Germain 22). This is a widely used domino theory is a very good theory of how they occur.

There has been much discussion lately about another important development from the domino

theory and that is unsafe acts and unsafe conditions. These are usually the superficial causes of

accidents. The main issue is to make sure that systemic causes and factors of accidents are

developed and analyzed. A good example is when a construction worker steps into a hole. Many

times in this analysis the cause is an unsafe act of the construction worker not paying attention,

while a real issue is why the hole was not guarded. An even higher-level analysis could be

supervision or budget issues.

Multiple Cause Theory Accidents are rarely caused by one act or condition. They are the result of many acts, conditions,

and causes (complex, simple, obvious, obscure, and systemic). The System Safety Development

Center from the Department of Energy stated

“When considering why an accident or incident occurred, more than one root

cause must be considered. Very seldom will just one root cause create a

condition that results in an accident. In most cases it requires a chain of root

causes that reaches from top management to the lowest level of the work process.

Correcting the specific root causes generally will only correct the bottom-level

conditions. Correcting the systemic root causes is more likely to correct all of

the root causes in a particular chain that reaches from management to the bottom

work processes. (SSDC ii).

If safety engineers only analyze only “acts and conditions” they will miss many

higher-level issues and this theory is based on Dan Petersons statement

Today we know that behind every accident there lie many contributing factors,

causes, and subcauses. The theory of multiple causation states that these factors

combine together in random fashion, causing accidents. If this is true, our

investigation of accidents ought to identify as many of these factors as possible—

certainly more than one act and/or condition. (Peterson 16).

Epidemiological Theory Another useful theory is the Epidemiological Triangle, which consists of the host (the person who

gets a disease), the agent that cause the disease (virus, bacteria, etc.), and the vehicle or

environment that carries the disease (mosquito, tick, water sources, etc.). This concept can be

applied to accidents when the host is the person injured, the agent is what did the injuring, and the

vehicle is what conveyed the agent. This is a simple diagram of an accident at shown in Figure 4.

Figure 4. Epidemiological Triangle.

Haddon Matrix

The Haddon Matrix is a theory of the factors and phases of injury. Each accident has a pre-

injury, injury, and a post-injury phase. During these three phases there are three factors that

influence the outcomes of the event. These events are the interactions between the human,

equipment, and the environment in each phase of injury. A sample Haddon Matrix is included in

Table 3.

Table 3. Haddon Matrix.

Technical/Engineering Approach

Technical or engineering approaches to accident theories are very specific and discover lower

level causes and system failures. They are excellent for discovering and investigating system or

equipment failures, but too narrow in scope for most other types of accidents.

Human Error/Human Factors Theory

There are many different types of human factors or human error theories. The basis of these

theories is to identify the human/machine/environment interface. The key is to determine if the

interface or interaction had an effect on the accident. The steps are to analyze how the human

interacted with the machine, equipment, environment, etc. The key is to focus on the work

environment that produces the bad behaviors and try to eliminate the behavior instead of focusing

on the human error (Oakley 36). One of the main parts to human error theory is to determine the

Host

Vehicle (Environment) Agent

HADDON MATRIX

PHAS

ES

FACTORS

Human Equipment Environment

Pre-Injury

Time pressure to

perform the job

(rushing job)

Oily boots

Rainy

Injury

Feet and hands

slipping on ladder

Distance to ground

(distance of fall)

Slippery ladder

Post-

Injury

Concussion

Ladder fell over on

top of employee

Emergency

medical response

late due to rain

type of error. Error of omission is when one forgets to do something or misses a step, which is

usually caused by a distraction or diversion. Errors of commission is when someone performs

incorrectly or does something wrong, which is usually a lack of training.

Sequence of Events

The last theory is not so much of a theory, as a way to visualize an accident. All accidents are a

sequence of events. This philosophy lends itself to using many of the other theories, such as

multiple causation and the domino theory. The concept of this is to develop scenarios and

sequence of events to develop accidents, and then try to use controls to prevent them from

occurring.

Why Proactive Safety? The key concept is to prevent accidents. No one wants to get hurt, but actions and conditions will

dictate an accident. All accidents are caused and there are many consequences of accidents. The

health and safety of personnel is the utmost priority, but other issues include functional capability

of the plant after loss, public image and reputation, financial well-being (loss of sales), and also

civil or criminal legal action.

Proactive safety is identifying hazards (both facility hazards and hazardous actions by

people). These hazards are the start of the accident sequence. Each hazard must be identified so

a thorough analysis can be performed to find the ways an accident can occur. The last step is to

control the hazards as shown in table 4. Proactive safety is analyzing systems and tasks to work

safely and developing a safety philosophy to work safe on every task. This philosophy will keep

systemic problems from becoming an accident sequence.

Table 4. The steps for Proactive Safety.

Systems Safety Approach The key to a systems safety or task safety approach is to analytically and methodically identify,

analyze, and control hazards before an accident occurs. The concept of conducting analyses is to

break down the system versus the job or task. Analyze the systems such as the piece of

equipment and look for hazards. Then break down the tasks, what is the process for obtaining the

wood, loading it into the machine, etc. Obtain all of the hazards for this task also. The next step

is to ensure the hazards versus failures. Many of the systems safety techniques find failures,

however to prevent accidents you must look for hazards.

Safety Motto

IDENTIFY HAZARDS

ANALYZE HAZARDS

CONTROL HAZARDS

Proactive Safety Techniques Using Accident Theories The key to all accidents is to uncover and analyze the accident sequence, determine the causal

factors, and find corrective actions that will prevent future accidents. After the hazards are

identified and analyzed, causal factors are developed. Using these accident theories, the causal

factors or what would have prevented the accident are used to develop the proper controls or

corrective actions. The theories should be used to validate and find systemic problems at all

levels. Management issues, worker issues, engineering issues (design), as well as policy issues

need to be analyzed. All levels need to be looked at to provide corrective actions and

accountability to prevent accidents. Telling a worker to work safer will only prevent that worker

from having a future accident. If the control is a policy issue or a design issue, then the corrective

action needs to be addressed at that level to fix future accidents. Proactive safety is a chance to

look at the failures in the safety program and fix them. (It is not the time to place blame or look

at human error. This is the time to look at what caused the human error.)

The next step to prevent accidents is to implement the corrective actions. All corrective

actions need to be tracked and a strict timetable established. If corrective actions are assigned to

a department or someone is accountable for the corrective action, then usually they will be fixed

in a more timely manner. All corrective actions or recommendations must be communicated

clearly and objectively. The last step is to conduct a follow up and make sure the corrective

actions are in place or working correctly to prevent accidents.

Compliance/Regulations One of the best ways to use the theories of accidents is to use the standards/regulations to find

hazardous situations. While standards are the minimal compliance, it is a great starting point.

When performing a walkaround look for potential accident sequences or use the OSHA

categories of accidents as listed in Table 5. These are the categories that would be marked for an

OSHA recordable, so if you alleviate these form occurring, then you dust stopped the domino or

sequence of events of an accident.

Table 5. OSHA Categories of Accidents.

Job Safety Analysis Everyone has probably performed the basic job safety analysis of listing the steps to a job,

documenting the hazards, and developing controls. A job safety analysis is an excellent proactive

safety approach. When conducting a job safety analysis, look at the potential dominos and

sequence of events for an accident. Look for negative paths and use the OSHA categories to

structure your hazards. Also use the unsafe acts/unsafe conditions to determine where lack of

control situations can occur.

OSHA Categories of Accidents

Struck By Caught In

Struck Against Fall, Same Level

Caught Between Fall to Below

Contact With Overexertion

Contact By Exposure

Contact On

While identifying the hazards is the hardest step of the job safety analysis, the most important

step is developing solutions to prevent the accident. The hazard control precedence was

developed to try to prevent the accident in the best possible way to ensure that the control is

fixed. The first step is to try to design out or get rid of the hazard, if that cannot be accomplished,

then to try to substitute for a less hazardous task or equipment. The next step is to try to use

guards and safety devices to reduce the hazard. The next step is to use administrative controls

and procedures to control the hazard. The last step is to use personal protective equipment to

guard the person from the hazard. This is extremely important in that you want to try to control

the hazard at the highest level (System Safety Society 1).

Barrier Analysis This is a simple analysis that is very good at locating hazards and controlling them. A barrier

analysis is fairly simple to perform – keep the hazard from the target. This type of barrier

analysis considers potential hazards, the potential targets, and assesses the adequacy of barriers or

other safeguards that should prevent or mitigate an accident (Spear 27). This analysis is

extremely useful because it produces a graphical chart. The outcome can graphically explain the

accidents failures and also find the barriers that need to be corrected or added to prevent

accidents. The approach to this technique is very simple and is listed in Table 6. There is a

hazard and a target. The barriers try to keep the hazard from reaching the target. The first step is

to identify the hazard and the target. The next step is to identify or brainstorm all of the barriers

to get a comprehensive list and documented on a form as shown in Table 7.

Performing a Barrier Analysis

1. Identify the hazard and the target

2. Identify (brainstorm) barriers and controls

3. Evaluate the intended function of the barrier

Table 6. The steps needed to perform a barrier analysis.

Barrier Analysis Form

Barrier Purpose of Barrier

Table 7. The steps needed to perform a barrier analysis.

The barrier analysis summary chart can be an excellent graphical chart that displays the

failures of barriers for the accident in an easy to read graphical format. This chart can be

generated easily from the worksheet and be very helpful in developing corrective actions to

prevent future accidents. An example of a barrier analysis summary chart is illustrated in Figure

5.

Figure 5. Barrier analysis summary chart is an excellent tool to show the barriers and what

needs to be corrected to prevent an accident.

Summary It is important to understand how and why accidents occur by looking at the many accident

theories. The accident theories are a great tool to use, not only for accident investigations, but to

try to prevent accidents from occurring. Proactive safety techniques are extremely useful in

identifying, analyzing, and controlling accidents. Simple techniques can be used to prevent these

accidents. It is important to understand the aspect of and impact of proactive safety and the true

reasons these tools and techniques are applied, which is to prevent accidents.

Bibliography

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Control Institute, 1985.

Ferry, T. Elements of Accident Investigation. Springfield, IL: Charles C. Thomas, 1978.

National Safety Council. Accident Prevention Manual for Business and Industry, 12th

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IL: National Safety Council, 2001.

Oakley, J. Accident Investigation Techniques: Basic Theories, Analytical Methods, and

Applications. Des Plaines, IL: ASSE, 2003.

Oakley, J. and S. Smith. “Ergonomic Assessment and Design: The Key to Back Injury

Prevention.” Professional Safety. Feb. 2000: 35-38.

Barrier Analysis Summary Chart

HAZARD

TARGET

BARRIERS

Fall

Plant Procedure

Fall Protection

Safety

Training

Worker

Peterson, D. Techniques of Safety Management. New York: Mcgraw Hill Book Company, 1978.

Spear, J. “Incident Investigation: A Problem-Solving Process.” Professional Safety. April 2002:

25-30.

System Safety Development Center. MORT Based Root Cause Analysis. DOE SSDC-27, 1989.

Systems Safety Society. Systems Safety Analysis Handbook, 2nd

Ed. Systems Safety Society.

1997.