Technique For Human Error Rate Prediction - THERP Methodology

THERP Methodology

The methodology for the THERP technique is broken down into 5 main stages:

1. Define the system failures of interest These failures include functions of the system where human error has a greater likelihood of influencing the probability of a fault, and those of interest to the risk assessor; operations in which there may be no interest include those not operationally critical or those for which there already exist safety counter measures.

2. List and analyse the related human operations, and identify human errors that can occur and relevant human error recovery modes This stage of the process necessitates a comprehensive task and human error analysis. The task analysis lists and sequences the discrete elements and information required by task operators. For each step of the task, possible errors are considered by the analyst and precisely defined. The possible errors are then considered by the analyst, for each task step. Such errors can be broken down into the following categories:

• Errors of Omission – leaving out a step of the task or the whole task itself
• Error of Commission – this involves several different types of error:
  • Errors of Selection – error in use of controls or in issuing of commands
  • Errors of Sequence – required action is carried out in the wrong order
  • Errors of Timing – task is executed before or after when required
  • Errors of Quantity – inadequate amount or in excess

The opportunity for error recovery must also be considered as this, if achieved, has the potential to drastically reduce error probability for a task.

The tasks and associated outcomes are input to an HRAET in order to provide a graphical representation of a task’s procedure. The trees’ compatibility with conventional event-tree methodology i.e. including binary decision points at the end of each node, allows it to be evaluated mathematically. An event tree visually displays all events that occur within a system. It starts off with an initiating event, then branches develop as various consequences of the starting event. These are represented in a number of different paths, each associated with a probability of occurrence. As mentioned previously, the tree works on a binary logic, so each event either succeeds or fails. 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. Below is an example of an event tree that represents a system fire:

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.

3. 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 judgement. 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

4. 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.

5. Recommend changes to the system and recalculate the system failure probabilities Once the human factor contribution is known, sensitivity analysis can be used to identify how certain risks may be improved in the reduction of HEPs. Error recovery paths may be incorporated into the event tree as this will aid the assessor when considering the possible approaches by which the identified errors can be reduced.