Accident investigations should pay more attention to
human factors, especially behaviour. "Over 90% of accidents
may be attributed, at least in part, to the actions or omissions of
When it comes to examples we are spoiled for choice and
unfortunately many of them are quite spectacular. They
include Three Mile Island (1979), The King's Cross Fire (1987), the
Herald of Free Enterprise disaster, the Union Carbide disaster at
Bhopal (1984), the space shuttle Challenger explosion (1986), Piper
Alpha (1988) and Chernobyl (1986). More recent examples
include Buncefield (2005) and the BP Deepwater Horizon Gulf Oil
But although most accidents result from human failure, accident
investigations do not probe these behavioural causes in anything
like sufficient depth. This is a serious omission. It ignores vital
evidence that could help prevent recurrence.
Of course investigations will look at those human failings
leading directly to the accident but all too often that's it.
Content to put the cause down to human error and mete out a
suitably proportionate punishment they will look no further. But in
the words of the HSE publication, HSG 245 (Investigating accidents
and incidents),2: "Investigations that conclude that operator error
was the sole cause are rarely acceptable. Underpinning the 'human
error' there will be a number of underlying causes that created the
environment in which human errors were inevitable. For example
inadequate training and supervision, poor equipment design, lack of
management commitment, poor attitude to health and safety."
So why are these underlying behavioural causes so often
neglected? Identifying them could be invaluable.
"Behavioural aspects are often difficult to understand and
investigate and the lead investigator simply may not have enough
knowledge of human factors," says Malcolm Cope, an ergonomics and
human factors senior scientist with the Health and Safety
Laboratory (HSL). "They are more likely to be somebody with
an appreciation of the job, eg. an engineer working on the process,
or a safety adviser employed by the organisation. The
temptation is to look at the accident in black and white terms and
to focus on the immediate cause."
Nor does it help that behavioural factors are not always easy to
"Very often psychological factors underlie behaviours and these
are difficult to measure in a validated way," says Mike Gray, a
principal specialist inspector with the HSE. Mike speaks from
experience, having used his background in ergonomics and human
factors to investigate many accidents.
Human failures are grouped according to the immediacy of their
consequences. Active failures have immediate
consequences and usually involve frontline workers. Examples would
include an electrician getting a shock after failing to isolate
high voltage equipment or an operator in a control room pressing
the wrong buttons and causing a chemical escape.
The second group of failures are termed latent
failures. These failures are made by those further
back from the frontline. Examples include designers failing
to design safe equipment or job procedures, and managers failing to
provide adequate training or enforce safety standards.
Human failures are also classified by type. The two main
types are errors and violations
and these are further sub-divided according to the details of their
nature (Table 1.)
Errors are unintentional whereas
violations are deliberate. Examples of
errors include misreading a display and as a result making a wrong
decision which leads to an accident.
Individual human errors should never be looked at in
isolation. Accident investigators must take the context into
account. HSE's Mike Gray gives an example. "An operator
standing at ground level was using a hand held control to operate
an overhead gantry crane. He pressed a button and the crane
moved towards him. In a panic he attempted to send it the
other way. It continued to move towards him. He was crushed
by the metal load and hospitalised."
At first glance this may seem a simple case of carelessness. The
operator pressed the wrong button with disastrous consequences.
This type of human error is termed a 'slip', ie. a failure in
carrying out the actions of a task.
Further investigation, however, revealed that the 'slip' was not
just an example of carelessness.
"It was a case of poor job design and poor equipment," explains
Mike. "The design, or lack of design, of the job made it all
too easy for the operator to become confused. There were several
cranes and not all of them reacted in the same way to the buttons
on the hand held control. For some of the cranes the
left/right buttons worked the opposite way. Where you stood
relative to the crane could also made a difference, the button
sometimes seeming to send the crane in an unexpected
direction. It might have helped had there been signs on the
cranes clarifying how they responded to the controls."
Whilst errors are genuine mistakes. Violations, on the
other hand, are deliberate. An example would be a worker
deliberately operating a machine without a guard, or a worker
deliberately not wearing personal protective equipment in order to
One case study3 describes how a worker lost his hand
when he became entangled in packing machinery. Eight machines
were enclosed behind a fence to protect workers from coming into
contact with them. On the day of the accident the worker had
entered the enclosure to clear a blockage while all the machines
were still running. Normally opening the door to get into the
enclosure would have switched off the machines but the worker had
deliberately overridden the door's interlock.
On the face of it the worker had defiantly disobeyed the
rules. However, it was not as simple as that. "Very few
people are wilfully reckless," says Phoebe Smith, HSL's principal
human factors specialist. "There are usually other reasons for
it. Often it is to get the job done quicker.
Supervision turning a blind eye simply rewards the behaviour
because it saves time and money until, of course, an accident
Which is what happened here. Opening the door to enter the
enclosure would have switched off all eight machines.
Overriding the safety interlock on the other hand kept them
running. This reduced downtime and increased productivity
even though it put workers at risk. Management tacitly
encouraged this behaviour by dropping in on the packaging area to
discuss production targets. Nobody condemned the practice of
overriding the interlocks.
Defeating interlocks appears to be a widespread practice.
A recent report4 identified some of the main reasons for
defeating interlocks on Computer Numerical Control (CNC) machines
(eg. machines such as lathes that can be programmed to work
Several factors made defeating interlocks more likely.
Firstly, there were negative attitudes on the part of the workers
about the need for interlocking. These were coupled with the
belief that they would not be injured because their experience and
common sense would protect them. These are termed
predisposing factors). Secondly, there was
often poor machine design, lack of training and confusion about the
legal requirements. These are termed enabling
factors. Finally, there was a lack of visible
management commitment to safety, a lack of enforcement or
disciplinary action and a positive production benefit. These
are termed reinforcing factors.
The study found one of the most significant of these factors was
poor machine design. Operators found guards to be impractical,
hampering their ability to do the job.
Boredom and complacency are yet other factors to consider in
general. They can lead to human error or, in the case of the
former, very high levels of frustration and violation. Ironically,
attempts to make jobs safer have in some cases increased the levels
of boredom. One example is that of airline pilots.
Automatic pilots mean that long stretches of a flight can be taken
with the pilots not actually flying the plane. This means
that they may not be mentally ready to take control in an
emergency. Also, the lack of the number of hours actually
piloting the aircraft may leave them rusty. Airlines now
specify that pilots have to fly the plane for a fixed number of
Dockworkers at Southampton docks rotate among three jobs;
driving cranes, driving container carriers and working on the
dockside and the ships. This mitigates the complacency that
could arise from doing just one job.
Mike Gray lists other factors to consider when looking for the
deeper causes of accidents:
"Although morale and complacency are difficult to measure they
can influence behaviour to make accidents more likely.
Another important factor is the culture within the organisation,
particularly the degree of management involvement and
Although management involvement and leadership are not directly
measurable, some of their manifestations are. For example,
does higher management tour the organisation to solicit the views
of the workforce on health and safety issues? Are there
policies outlining health and safety expectations and are these
policies communicated and, if necessary, reinforced ? Our
colleagues in HSL have developed a straight forward survey to
measure safety culture and identify where the issues
And, of course, poor training can lead to wrong behaviours,
particularly in an emergency. Mike Gray gives one
example: "There was a fire in a care home. When the
lights flashed on the fire alarm panel staff did not know what to
do. They wasted time trying to understand what the flashing
lights meant instead of evacuating the patients. The issue
here was a lack of understanding of how the fire alarm
worked. This was a training failure."
Violations are often prompted by poorly written
procedures. HSL's Malcolm Cope comments: "They can be hidden
away in impenetrable manuals whereas they should be integrated into
An increasingly popular way of doing this is using the concept
of "nudge". As its name suggests, this is a subtle
intervention designed to nudge you. And when used in a health
and safety context the nudge is a nudge towards the right kind of
A very simple example would be footprints on the floor to
encourage you to use the correct walkway. Another example
could be to paint the shape of a tool on the wall just below the
hook on which it should hang to encourage individuals to put tools
back in their correct place. Taken together these nudges
could constitute a gentle shove towards a safer working
These examples highlight just some of the individual, job and
organisational factors that can influence the safety behaviour of
Which are the most important?
In its publication, HSG 48, HSE is in no doubt. "Organisational
factors have the greatest influence on individual and group
Accident investigators, therefore, cannot ignore them.
"Organisational factors set the context in which everything else
happens," explains Mike Gray. "One of the most important
organisational factors of course, is leadership. Leadership
sets the expectations. It sets the culture and it is hard for
an individual to stand against a prevailing culture whether good or
bad. If effective leadership is not there it can be difficult for
those lower down to implement other changes because they are always
working against the top. When investigating an incident I
principally look for things that people did not get right.
For example, were managers deliberately ignoring things being done
Clearly it is difficult to overstate the influence of health and
safety leadership, whether it is the manager of Arsenal football
club berating one of his players for smoking, or a managing
director setting an example by wearing a hard hat on site.
Leaders set the expectations.
Care has to be taken, however, that this expectation does not
get lost or mistranslated as it travels down the
"In many organisations, managers at the top can be saying all
the right things, but the message somehow gets lost or changed by
the time it has filtered down to the bottom," says Phoebe
Smith. "For example, it can be transformed into subtle hints
suggesting a production verses safety situation. This can
lead to unspoken pressure to take shortcuts."
Accident investigations should therefore involve workers.
Not only will their expertise of the workplace and job help to
determine the causes of the accident; they can also provide a
comparison between safety culture at the top and the bottom of the
The management tours mentioned earlier can also ensure the
safety message does stretch from the top to the bottom of the
organisation. They may also help create another powerful
force for safety; peer group pressure. Organisational culture
has been described as the way we behave when no one is
watching. That culture will be deemed a success when, for
example, a worker on a night shift tells his colleague he should be
wearing his safety spectacles.
And, of course, the degree of worker involvement is yet another
indicator of an organisation's safety culture. Had workers
been involved in discussions about the overhead crane accident
described on page??, job redesign may have prevented the
Jane Hopkinson, HSL psychologist and co-author of the research
report into why interlocks are overridden4) is currently
working on HSL's Behaviour Change toolkit - ACT. The toolkit
emphasises worker involvement from the beginning. It stresses the
importance of leadership, ensuring the environment is safe and that
there is a good safety culture in place. She sees worker
involvement as one of the biggest weapons against cynicism. The
toolkit is currently being evaluated and will be available to
organisations from next March.
Handling human error
Finally, the way investigators handle investigation is of course
a human and behavioural factor in its own right, especially where
human error is concerned.
Knee-jerk blame will almost certainly be counter-productive
because it will undermine workers' trust and create an atmosphere
of suspicion in which they will be reluctant to be open about what
happened. And once that trust has gone a vital source of
knowledge will be lost to the organisation.
Reducing error and influencing behaviour, HSG 48
(second edition 1999), Health and Safety Executive. Although,
at 72 pages, this publication is weighty, it could be used as a
comprehensive checklist for accident investigators looking to
include human factors in their work.
Investigating accidents and incidents. A workbook for
employers, unions, safety representatives and safety
professionals, HSG 245 (2004, reprinted with amendments 2011).
Ergonomics and human factors at work, a brief guide,
INDG90, HSE. In effect a cut down version of HSG 48 for
SME's. This publication is aimed at employers and
Identifying the human factors associated with the defeating
of interlocks on Computer Numerical Control (CNC) machines.
Research Report (RR) 974. Prepared by the Health and Safety
Laboratory for HSE, 2013. HSL.