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Practice meets perfect
May/June 2023 issue

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T is for Therp

Open-access content Thursday 14th November 2019
From the archive:  Just so you know, this article is more than 3 years old.

However, in an emergency -- a co-worker has a limb stuck in the machine, say -- when people are shouting, the operator presses the wrong button. A simple change such as colouring one button red would make the error less likely. But by how much?

With more complex displays and controls, on which changes would be more expensive, how would you calculate the benefit of any alteration to the user interface? How can you decide which of two equally expensive changes would provide the best benefit?

This was a dilemma that the nuclear industry faced after the part-meltdown of a reactor at Three Mile Island, Pennsylvania, in 1979. In response, the US Nuclear Regulatory Commission produced the Handbook of Human Reliability Analysis (Swain and Guttmann, 1983). In its 728 pages (bit.ly/32Zh1zl) it attempts to put numbers on the likelihood of different user errors, given different types of job aids, and to describe the performance-shaping factors (PSF) that can cause the error probabilities to be better or worse.

From this analysis, the technique for human error-rate prediction (THERP) was born. Two years later, Jerry Williams published proposals for human error assessment and reduction technique (HEART) based on his work in the UK energy sector. HEART gave greater consideration to a wider range of PSF, including inexperience on the part of the operator, and overload of information.

THERP and HEART provided analysts with evidence-based human error probabilities (HEPs). The human components of a system could be shown in an event tree, alongside failures of valves, alarms and response systems. Decisions about user interface design, procedures and job aids could be based on calculations rather than guesswork.

To apply THERP (or HEART) there must first be a task analysis (see IOSH Magazine September 2018: bit.ly/2kJyriW). At each node of this process, the types of human error possible need to be defined. For example, following a smoke detector alert, the security guard's task is to go to the location to check for fire. Errors could include: doing nothing (omission); going to the wrong location (selection); going to the right location but too slowly (timing); or immediately phoning the fire brigade without checking (sequence). Other error categories include quantity (doing too much or too little of something) and extraneous acts (doing something unrelated).

If you want to know more about THERP, HEART and other human error assessment techniques that followed, the HSE Research Report 679 Review of Human Reliability Assessment Methods (2009) has further references ( bit.ly/2kzRcFB).

Techniques such as THERP and HEART have their limitations. Swain and Guttmann admitted there was a "paucity of actual data on human performance" on which to base the HEPs. The time required to apply the techniques is unlikely to be practicable in most occupational health and safety responses.

However, some generalised features of THERP are useful to understand when carrying out a risk assessment. The first is the importance of understanding the task. If the team considering 'changing a fuse' all have a different idea of what the task involves, the assessment will be unreliable. If there isn't the resource for a full-blown task analysis, a clear description of the task being studied would improve many assessments.

A second lesson from THERP is to understand what makes errors more (or less) likely. For example, performing rule-based actions when written procedures are available and used could be 100 times less error-prone than performing the same action when written procedures are not available. You are ten times more likely to leave a step out of a procedure than to add a step in, so instructions that remind people what they should do will be more effective than ones telling them what not to do.

The final lesson to take from THERP and HEART is a change of mindset. Too much behavioural safety literature identifies the correct behaviours as something the worker can choose to do, or not to do.

THERP reminds us that for every physical or cognitive task there is a probability of error. It also reminds us that reducing -- or increasing -- the probability of those human errors is the responsibility of those who design the physical and psychosocial environment in which people work.

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 U is for underlying cause

Monday 9th December 2019
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?
Open-access content

 S is for SRK (Skill, rule, knowledge)

Sunday 13th October 2019
Forty years ago, the Danish engineer Jens Rasmussen published his SRK framework, distinguishing between skill-based, rule-based and knowledge-based behaviours. 
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 V is for violation

Tuesday 7th January 2020
In J is for Just Culture, the definition for the term was given as “a culture in which frontline operators and others are not punished for actions, omissions or decisions taken by them which are commensurate with their experience and training, but where gross negligence, wilful violations and destructive acts are not tolerated” (see IOSH Magazine, January 2019: bit.ly/37jtUXN).
Open-access content

 R is for risk homeostasis

Monday 2nd September 2019
In his 1879 publication Notes on Railroad Accidents, Charles Francis Adams describes how a rail company chose not to implement a block protection system because it feared that “those in charge of trains and tracks, who have been educated into a reliance upon it under ordinary circumstances, will, from force of habit if nothing else, go on relying upon it, and disaster will surely follow”.  
Open-access content
Image credit: ©iStock/francescoch

 W is for WYLFIWYF

Thursday 27th February 2020
In the days when most computers had black screens with green text, the term ‘What You See Is What You Get’ (WYSIWYG, pronounced whizzy-wig) referred to new computers that were being developed by companies like Apple and Xerox, where documents appeared on the screen as they would be printed.
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 X is for gens X, Y and Z

Wednesday 4th March 2020
While it is no longer acceptable to assume that all men are stronger than all women, or that people of one colour have different personalities to those of another colour it is, it appears, entirely acceptable to declare that anyone born since 1980 is addicted to social media and will ‘challenge traditional hierarchical HSE systems’, while anyone born before that date is a luddite with no understanding of the modern age, but will be quite happy to toe the line.
Open-access content
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