Accident Causation Models: Comparison of Heinrich’s Model & the Bow-Tie Model Research Paper

Introduction

In the context of occupational health and safety (OHS), one of the most important aspects is the analysis and prevention of accidents. In terms of OHS, an accident might be defined as an unexpected event with undesirable outcomes that results directly or indirectly from human activity and not from natural events or disasters (OHS Body of Knowledge, 2012). The goal of safety management is to eliminate accidents or at least to reduce the number of workplace accidents. In order to achieve this goal, it is important to study the mechanisms causing accidents to happen.

The role of accident causation models in the OHS framework is illustrated by Fig. 1. It is possible to see that accident causation models are essential both for maintaining workplace safety and health.

Figure 1. OHS framework (OHS Body of Knowledge, 2012)

There exist various accident causation models that might be used for OHS purposes. These models can be divided into three key groups: simple linear models, complex linear models and complex non-linear models (Hollnagel, Woods & Leveson, 2007). The purpose of this report is to compare and contrast two accident causation models. The models selected for comparison are Heinrich’s causation model, also referred to as the domino model, and the bow-tie model. The paper is structured in the following way: the introduction is followed by the descriptions of the selected accident causation models. The models are further compared in terms of their approach, theoretical background and methodology. The next section explores the areas of disagreement between the models, and the following section is devoted to the critique of the selected models in terms of reliability and validity. The conclusion summarizes the findings and key points.

Main Body

Heinrich’s Model

Accident causation model was developed by Herbert W. Heinrich in 1931 (Channing, 2013). This model is also referred to as the “domino theory”. This model belongs to simple linear models. According to Heinrich’s model, an accident represents a chain in a series of events that occur in a fixed order. Heinrich outlined five accident causes and related factors that are lined up in a sequential order: social environment/ancestry, fault of the person, unsafe acts or conditions, accident and injury (Ale, 2009).

Fig. 2 illustrates Heinrich’s model. This model is also called the “domino model” since the factors in this model interact like chained dominos: when one of the factors comes into action (falls off metaphorically), the next elements of the logical chain are also activated (Channing, 2013).

Figure 2. Heinrich accident causation model
(Carrillo-Castrillo, Rubio-Romero & Onieva, 2013)

The Bow-Tie Model

The bow-tie model is also called the cause-consequence diagram. It was initially developed by Shell for the purposes of risk assessment (Hale et al., 2007). The model has a central point (undesirable event), with the events and circumstances preceding the undesirable event shown on the left hand, and various scenarios and consequences of the undesired event shown on the right hand. Fig. 3 illustrates the bow-tie model. This model shares the understanding of accidents causation with the Swiss Cheese model (Hale et al., 2007). The bow-tie model belongs to complex linear models as it reflects the combinations of various accident causes and consequences of critical events.

The key concepts of the model are hazards (acting on the left hand), consequences (acting on the right hand) and top events (central points of bow-tie chart). It should be noted that top events in this model are events with significant potential for unwanted consequences (Ferdous, 2011). The purpose of safety management in this context is to create barriers in order to prevent the emergence of negative consequences after top events, or to prevent top events (Ferdous, 2011).

Figure 3. Bow-tie accident causation model (Channing, 2013)

Comparison of Models

Theoretical Background and Approach

The models have one generic similarity in their approach to viewing accident causation: both Heinrich’s model and the bow-tie model represent the accident as a consequence of initial events and circumstances, followed by the accident and its negative consequences (e.g. injury). The bow-tie model shows the hazards preceding the top event on the left hand, and the domino model also contains three elements preceding the accident. Furthermore, the domino model lists injury as the consequence of the incident, and the bow-tie model also deals with consequences.

At the same time, there exist notable differences between the models in terms of approach. These differences are caused by the different view of accident causation. The domino model relies on the idea that the key accident causes can be classified into three major groups – social environment, fault of the person, and unsafe act or condition. Furthermore, Heinrich assumed that these factors are connected in a linear logical chain and that preventing one “domino” from falling down allowed to prevent logical activation of the next elements.

These assumptions contrast with the assumptions of the bow-tie model. The latter model relies on the assumption that there exist multiple events and circumstances that might activate various existing hazards. The bow-tie model assumes that different causes are not linked in a logical order and might happen independently. In other words, the bow-tie model assumes that there are multiple entry points causing top events to happen, and that addressing several causes might not be enough to maintain OHS, since hazards might be activated by other events and circumstances.

Methodology

In terms of methodology, both models agree that in order to achieve workplace safety, it is necessary to introduce barriers between accident causes and the events leading to accidents. In addition, both models might be used both for assessing and reducing OHS risks, and for improving workplace safety (Katsakiori, Sakellaropoulos & Manatakis, 2009). One more common methodological characteristic of both models is their general-purpose nature. Both models might be applied in different environments and in different industries (Katsakiori, Sakellaropoulos & Manatakis, 2009). The common limitation of these models is the focus on specific events and causes instead of viewing the whole organization and work process from more systemic point of view (Hollnagel, Woods & Leveson, 2007).

At the same time, the scope of application for these models is different. The domino model is most frequently used for analyzing incidents, detecting critical elements in the process chain and developing multifaceted OHS improvement programs. The bow-tie model is most often used for evaluating risks, devising risk reduction strategies, assessing the effectiveness of OHS strategies and promoting OHS compliance (Hale et al., 2007). The bow-tie model is efficient for analyzing various accident scenarios and evaluating risks for particular scenarios and for specified top events in general (Ferdous, 2011).

Critique

Critique of Heinrich’s Model

Heinrich’s model is time-efficient and useful for investigating accident causes. However, it has certain disadvantages in terms of reliability and validity. First of all, there is one key reliability weakness – the assumption of linear relationship between three categories of accident causes. In more complex situations this assumption does not hold (Manuele, 2003). Secondly, the domino model does not account for the influence of external factors which represents a validity threat. The domino model might therefore be not applicable for devising accident prevention strategies due to these weaknesses. However, for simpler processes that have low dependence on external factors the domino model might be the most efficient approach.

In order to overcome validity weaknesses, McLoughlin suggested adding a sixth domino to Heinrich’s model (HSM, 2010). McLoughlin stated that “external factors” domino should be added in the beginning of the chain. The suggested change allows accounting for global factors such as recession and economic changes in the context of OHS strategy (HSM, 2010).

Critique of the Bow-Tie Model

The bow-tie model has several advantages in the context of OHS analysis and accident prevention. This model combines the analysis of accident causation and risk assessment scenarios. Furthermore, it can be used for devising control measures aimed at preventing undesirable events, and for addressing the consequences of undesirable events by introducing recovery measures (Manuele, 2003). However, the bow-tie model has a significant limitation in terms of validity as the model treats accidents separately. Common factors influencing the whole work process might be missed using such approach.

Furthermore, the model is generic so the specific causes, top events and effects should be determined individually for every particular case. If some critical accident causes or top events are missed, the reliability of the model will be weakened. At the same time, the bow-tie model is well-applicable at the departmental and organizational level, since it accounts for multiple causes and multiple accident scenarios (Ale, 2009).

Conclusion

The purpose of this report was to compare and contrast two accident causation models – Heinrich’s model and the bow-tie model. Heinrich’s model belongs to simple linear causation models, while the bow-tie model belongs to the group of complex linear models. Both models identify factors preceding the accident, the event itself, and the consequences of the accident. The domino model assumes that different groups of factors are related to each other in a linear causal manner, while the bow-tie model accounts for multiple incident causes and hazards, and for multiple scenarios of accident consequences.

The domino model is more applicable to simpler processes that are not affected by external factors. Heinrich’s model does not account for external factors, and therefore has a weakness in terms of validity. In addition, the domino model is efficient for exploring accident causes. The bow-tie model is more appropriate for analyzing accidents at higher levels, assessing risks and evaluating OHS performance and compliance. However, since the bow-tie focuses on top events, it lacks a broader view and therefore might have weaknesses in terms of validity. In general, these two models have different assumptions and should be used in different situations, and might be combined for investigating accident causation. The bow-tie model is more appropriate at the departmental or organizational level, while the domino model might be useful at the process level.

 

Reference List

Ale, B. (2009). Risk: An Introduction: The Concepts of Risk, Danger and Chance. Routledge.

Carrillo-Castrillo, J.A., Rubio-Romero, J.C. & Onieva, L. (2013). Causation of severe and fatal accidents in the manufacturing sector. International Journal of Occupational Safety and Ergonomics, 19(3), 423-434.

Channing, J. (2013). Safety at Work. Routledge.

Ferdous, R. (2011). Analyzing system safety and risks under uncertainty using a bow-tie diagram: An innovative approach. Process Safety and Environmental Protection, 91(2), 1-18.

Hale, A.R. et al. (2007). Modeling accidents for prioritizing prevention. Reliability Engineering and System Safety, 92(12), 1701-1715.

Harms-Ringdahl, L. (2013). Guide to Safety Analysis for Accident Prevention. IRS Riskhantering AB.

Hollnagel, E., Woods, D. & Leveson, N. (2007). Resilience Engineering: Concepts and Precepts. Ashgate Publishing, Ltd.

HSM. (2010). Domino theory updated. Health and Safety Matters. Retrieved from http://content.yudu.com/Library/A1r2o6/HealthampSafetyMatte/resources/62.htm

Katsakiori, P., Sakellaropoulos, G. & Manatakis, E. (2009). Towards an evaluation of accident investigation methods in terms of their alignment with accident causation models. Safety Science, 47(7), 1007-1015.

Manuele, F.A. (2003). On the Practice of Safety. John Wiley & Sons.

OHS Body of Knowledge. (2012). Models of Causation: Safety. Safety Institute of Australia Ltd.

 

 

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[Accessed: June 1, 2020]

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