U is for underlying cause

In C is for Causality we questioned the nature of causality. When Fred trips on building materials across the path while making a phone call, did the obstacle cause the trip or was Fred’s inattention the cause?

We concluded that “for convenience” we consider “multiple causes, usually categorised as immediate, underlying and root causes”.

This distinction between three types of causes appears in many occupational health and safety courses, including those run by IOSH. Definitions are given in HSG245, Investigating Accidents and Incidents, published by the UK Health and Safety Executive (HSE). Although the immediate cause is “the most obvious reason why an adverse event happens, e.g. the guard is missing” and the root cause is the “initiating event or failing from which all other causes or failings spring”, the underlying cause sits somewhere between. It is “the less obvious ‘system’ or ‘organisational’ reason for an adverse event happening”. 

The HSE gives examples of underlying causes, from the failure to make machinery checks (which are quite close to the immediate event) to production pressures (which are much closer to a root cause).

A common approach to incident investigation is the ‘five whys’:

  • Why did Alan lose his toe? Because his foot was stuck in moving machinery.
  • Why was his foot in moving machinery? Because he climbed on top of it, slipped and no one could turn the machine off.
  • Why was the machinery moving?
  • Why did the check need him to climb on top of the machinery?
  • Why was there no emergency stop button?

The immediate cause – the movement of a blade across Alan’s foot – is obvious. The underlying causes are more complex. It is not a simple chain of cause and effect: that his foot was in moving machinery opens up the possibility of asking yet more whys. Except for simple incidents, five whys won’t reach a root cause.

HSG245 states: “It is only by carrying out investigations which identify root causes that organisations can learn from their past failures and prevent future failures.” The difficulty here is how to determine at what point a root cause has been identified. 

David Ramsay has specialised in accident investigation using root cause analysis for more than 30 years. As a result, he is clear about determining the root cause and, by implication, defining the underlying causes. 

The root cause is the most fundamental cause that can be resolved by management


“The root cause is the most fundamental cause that can be resolved by management,” he says. “One could go on and on, for example blaming the parents of someone who made an error in design. Clearly this is nonsense. The investigator has to decide when there is no value in going further. Management can’t change the weather, but management can assess the likelihood of flood and prepare appropriate defences.”

Ramsay’s comments suggest that identifying a causal factor as an underlying or root cause depends on the skill of the investigating team, whether local or head office driven. At a more superficial level, a hole in the ground causing a fall could be attributed to the underlying causes of a poor inspection and maintenance regime. Good root cause analysis should question the specification of the floor and if it is appropriate to the activities being completed there, for example, what is causing the holes?

Duncan Spencer, head of advice and practice at IOSH, suggests: “It is important not to stop asking why until you are sure that you are truly examining systems design, local culture or specification of environment and equipment.”

Confusion about the difference between an underlying and root cause is perpetuated by the HSE guidance using the terms interchangeably in HSG245 and mistakenly labelling some aspects of the examples provided. Spencer says: “The correct use of these words is important if we are to create truly preventative working practices.”

IOSH courses acknowledge these three terms, but in practice it is important that OSH professionals get into the habit of asking why until they go beyond mere failures in operational practice and the application of systems, adds Spencer. 

In Safety Myth 101, Carsten Busch seems to agree. In discussing the application of terms such as immediate, underlying and root cause, he writes: “The causes and their labels are nothing real.” He explains that the only useful distinction is between “direct causes (the causes directly leading to an accident) and all other causes (that came before those), which one might call underlying causes”. 



Bridget Leathley is a freelance health and safety consultant, providing risk management support in facilities, retail and office environments.  She delivers face-to-face safety training including IOSH and bespoke courses, and contributes to e-learning courses through evaluations and design work.  She has been writing for health and safety publications since 1996.  

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