Regional Basic Professional Training Course in Korea

11. Human Performance: A Perspective on Human Factors
It starts off with what is known as an initiating event and then branches are developed as
various consequences of the starting event are added. These are represented in a number
of different paths each of which is associated with a probability of occurrence. As was
previously mentioned, the tree works on a binary logic so that each event either has
success or failure. With the addition of the probabilities for the individual events along
each path i.e. branches, the likelihood of the various outcomes can be found. Therefore,
under the condition that all of a task’s sub‐tasks are fully represented within a HRAET,
and the failure probability for each sub‐task is known, this makes it possible to calculate
the final reliability for the task.
Estimate the relevant error probabilities. HEPs for each sub‐task are entered into the tree;
it is necessary for all failure branches to have a probability otherwise the system will fail
to provide a final answer. HRAETs provide the function of breaking down the primary
operator tasks into finer steps which are represented in the form of successes and failures.
This tree indicates the order in which the events occur and also considers likely failures
that may occur at each of the represented branches. The degree to which each high level
task is broken down into lower level tasks is dependent on the availability of HEPs for
the successive individual branches. The HEPs may be derived from a range of sources
such as: the THERP database; simulation data; historical accident data; expert judgment.
PSFs should be incorporated into these HEP calculations; the primary source of guidance
for this is the THERP handbook. However the analyst must use their own discretion
when deciding the extent to which each of the factors applies to the task
Estimate the effects of human error on the system failure events. With the completion of
the HRA the human contribution to failure can then be assessed in comparison with the
results of the overall reliability analysis. This can be completed by inserting the HEPs
into the full system’s fault event tree which allows human factors to be considered within
the context of the full system.
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