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, 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.

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