BOS 4601 Unit II Assessment

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Why is it better to apply the multiple causation theory rather than the unsafe acts/unsafe conditions model to an accident investigation? Provide an example that illustrates your point. 

The concept of unsafe acts / unsafe conditions was introduced by Heinrich’s 1931 original theory of accident causation (Oakley, 2012). When utilizing that model, an investigator would likely be able to determine what course of events led to the accident in question, but may not necessarily determine why the accident occurred. When focusing on the, what happened that led to this style of investigation, the end result could potentially miss other opportunities to prevent such an accident from occurring again. For years, investigators were trained under the Heinrich’s dominoes premise that 88% of occupational accidents are caused by a worker’s unsafe act(s) (Manuele, 2014). In recent times, it has become apparent that investigators trained under this model may have been stopping short once they identified the worker’s unsafe act as they would simply stop looking for additional information; thereby possibly missing other causations.

When investigators apply the multiple causations theory, they are not only able to answer the, what happened that led to this question, but they are also able to answer the question as to why something happened that led to this. As such, they are more suited to understand all factors that contributed to an accident and can better prevent further occurrences by fixing these issues. Management Oversite and Risk Tree (MORT); root cause analysis, developed by the US Department of Energy, is in an example of multiple causation theory (Oakley, 2012).

In short, contemporary research indicates that discovering all of the causal factors of an accident is paramount to preventing accidents in the future. By focusing on acts and conditions alone, investigators can miss higher level issues that have not been addressed (Oakley, 2012). An example of this short-sidedness was discovered in excerpts from the Deepwater Horizon explosion report. Researchers taking an additional look at the 2010 accident found investigators concentrating solely on worker’s unsafe acts while inadvertently missing potential additional causal factors built into the work systems (Manuele, 2014).


Manuele, F. A. (2014). Incident Investigation Our Methods Are Flawed. Professional Safety59(10), 34–43. Retrieved from:

Oakley, J. S. (2012). Accident investigation techniques: Basic theories, analytical methods, and applications (2nd ed.). Des Plaines, IL: American Society of Safety Professionals.

What is the difference between linear and non-linear accident models? Why are non-linear accident models not used more often in workplace accident investigations? 

The history of accident models go back to the 1920’s with three (3) distinct models; Simple Linear Models, Complex Linear Models, and Complex Non-Linear Models (Holnagel, 2010). In essence, simple linear model relates to s sequence of unsafe acts or conditions led up to the accident. If one of the unsafe acts or conditions were removed, the accident would not have happened. An example of this model is the classic domino theory; one domino (act or condition) falls into another which causes the second to fall and so on. In simple linear terms, should one of those dominos, to include the first, not be in place, the accident would not have occurred.

The complex linear model is also sequential and is based on the assumption that accidents are a complex combination of multiple unsafe acts or conditions that result in an accident. By taking action prior to the sequence beginning, via safety features/measures, the accident can be prevented (Hudson, 2014). This model has the presumption that unsafe acts at the start of the event could have been prevented, but that by the time of human involvement, near the end of the path already in motion, the accident has begun. The claim is that by focusing on strengthening defenses prior to human involvement, and thereby removing an unsafe element, accidents can be avoided (Hollnagel, 2010). Both simple and complex linear models assume there is a singular root cause of an accident.

Complex non-linear models revolve around the assumption that several factors, working in concert and at the same time, result in an accident occurring. This model suggests that multiple causes may be in play and that each of these causes would need to have been corrected for the accident to not have taken place (Hudson, 2014).


Hollnagel, E. (2010). Models of Causation. FRAM Background. Retrieved from:

Hudson, P. (2014). Accident causation models, management and the law. Journal of Risk Research17(6), 749–764. Retrieved from:

Oakley, J. S. (2012). Accident investigation techniques: Basic theories, analytical methods, and applications (2nd ed.). Des Plaines, IL: American Society of Safety Professionals.

A maintenance worker mopped the floor in a hallway and then left the area without posting a “wet floor” warning. The lighting in the hallway had been lowered to conserve energy, so visibility was poor. Two employees walked down the hallway on their way to lunch, discussing where they wanted to go to eat. One of the employees slipped and fell, suffering a broken wrist. 

Discuss how you could apply a domino theory to investigate this accident.

In this scenario, the accident would be classified as a major injury case; a recordable injury resulted after an accident and that injury requires the attention of a licensed medical professional (Oakley, 2012). Utilizing Heinrich’s domino theory, the final domino would be the fall that resulted in the broken wrist. The first domino would be the environment where this took place. In our scenario, the lighting was lowered to conserve energy at time where employees, to include the maintenance worker, were working. Anytime lighting is lowered in this type of commercial / business environment, care must be considered to ensure staff or the general public will not be entering without warning of the same.

The next domino is this situation revolves around fault/person. While the maintenance worker likely had reason or even permission to perform mopping at that place and at that time, policy & past practice requires the area be posted with “Wet Floor” signage. That said, the investigation would also include answers to the questions regarding times when mopping is permitted and in what areas.

Domino three would represent the unsafe act itself and would include confirmation that the maintenance worker had been trained in the proper mopping procedures. In addition to the act of mopping, but the posting of the area as wet, as well as, the equipment and cleaning solutions utilized. A thorough investigation would check that cleaning chemicals that might lessen slip, if available, were used and if not, an answer as to why not. Once the mopping was completed, the investigator would need to know if the WET FLOOR signage was available and where it was located. Answers to the question if someone, other than the maintenance staff member moved them, took them away, and/or intentionally hid the signs need be collected.

Next in the investigation would be the unsafe condition(s) as represented as the fourth domino. Because the floor was wet and, conceivably only wet for a certain amount of time, was there a policy or procedure that needs to be addressed as to leaving that wet area unstaffed? Should the investigator recommend an adjustment that would ensure not only the WET FLOOR signage, but physical presence must be maintained until dry? Are there other options, in regard to cleaning chemicals, that are not currently be used that the organization could turn to as an alternative?

Finally, the last domino was the slip & fall that resulted in major injury to an employee. This action was foreseeable and preventable. Had the maintenance worker used the correct signage and/or stayed behind when he/she did not erect the sign, he/she could have warned the employees to walk elsewhere. Had the organization created a policy or directive so as to only perform the energy saving light-dimming after workers had gone home and not when cleaning of the floors was happening, conditions would have been safer. Reviewing training records or even providing additional training of maintenance staff and employees who may happen upon an area being cleaned or worked on should be considered. In short, there are several opportunities to learn from this preventable accident and come out of it better than the organization was prior to the accident (Oakley, 2012).


Oakley, J. S. (2012). Accident investigation techniques: Basic theories, analytical methods, and applications (2nd ed.). Des Plaines, IL: American Society of Safety Professionals.

The Haddon Matrix Theory involves the use of three (3) phases when investigating an accident; Pre-Injury, Injury, and Post-Injury. The investigator divides each of these phases into categories to represent what likely led to the accident happening. The factors include; Human, Equipment, and Environmental (Oakley, 2012). It is a way to graphically correlate the factors and phases of injury so as to provide the investigator with a visual reference of the information he/she is taking in. The matrix, once completed and filled in, isolates and compares interactions, development causes, and recommended corrective actions or responses (Oakley, 2012).

 Human FactorEquipment FactorEnvironment
Pre-InjuryTraining or Lack of Training for this job.What work was to be performed?Situational Awareness / Understanding.Replacement Water Valve Parts.PPE to include Breathing Equipment on-hand.Vault Emergency O2Manhole Cover, Ladder, Warnings/Signage. Manhole Cover, Ladder, Warnings/Signage.Vault Ventilation.Exterior Warnings/Signage.Environmental Dangers Training.
InjuryInitially one (1) overcome and collapsed.Second went in w/o PPE and also collapsed.Both Sustained Fatal Injury.PPE UsagePPE FailureVault Emergency O2 Failure.Warnings Visible & Ignored.Ladder Function.Opened Manhole Cover.Entry Made. Climbed Down Ladder.Collapsed Bottom of Ladder (x2).Fatal Injuries.
Post-InjuryFatality x2No EMS on-scene or called until workers both found deceased.PPE UsagePPE FailureVault Emergency O2 Failure.Warnings Visible & Ignored.Ladder Function.Second worker went in when first collapsed. No 911 call made prior to going in to aid partner.EMS not notified until too late.

The Haddon Matrix above would be utilized to better categorize each factor into their respected phases. Once completed, the investigator can develop causes, compare actions, and recommend adjustments going forward (Oakley, 2012). In this scenario, the two workers either did not have, or they did not understand, the training regarding entering confined spaces. Furthermore, once one went down without the required PPE and was overcome, the partner did not call for EMS. Rather, he/she entered as well and was also overcome. Investigation would include checking warnings and signage, training records, available PPE, and policy regarding the importance of summoning EMS prior to trying to assist others. The accident was 100% preventable and avoidable. Contributing factors and lack of training would need to be assessed prior to allowing further employees from attempting this task until the organization is confident all have been corrected.


Oakley, J. S. (2012). Accident investigation techniques: Basic theories, analytical methods, and applications (2nd ed.). Des Plaines, IL: American Society of Safety Professionals.

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