THERP

THERP — Technique for Human Error Prediction Rate

One method for analyzing human reliability is a straightforward extension of probabilistic risk assessment (PRA)—in the same way that equipment can fail, so can a human make mistakes and slips. One technique for predicting human error rates is the THERP (Technique for human error-rate prediction), which was developed in the 1950s. As with other PRA techniques, THERP models can use either point.

A THERP analysis considers different types of error, such as not following an instruction, choosing a wrong switch or skipping a step in a sequence of activities, and forecasts the error rate for each of these tasks. If a person can make more than one type of error when carrying out a task, then the probabilities are added to one another. For example, when opening a valve an operator may:

  • Open the wrong valve
  • Skip the step altogether
  • Open it only part way.

If the respective probabilities for these errors are 0.01, 0.03, and 0.03, then the overall error rate is 0.07 (excluding second-order terms). It is also possible to factor in recovery rates. For example, if the wrong valve is selected, then there may be a 40% probability that the operator will recognize and correct the error while there is still time, thus reducing the overall probability of error to 0.6×0.01 or 0.006.

A THERP analysis is most effective when the tasks are routine and when there is little stress.

Further information to do with THERP is provided at the post OSHA’s PSM — What’s Not There. Part 1: Human Error.

Publications at this site that provide information to do with THERP are shown below.