Full Name 

Technique for Human Error Rate Prediction

 

Origins of the tool

THERP was developed by Swain (when working for Sandia National Laboratories). Swain & Guttmann (1983) then prepared the THERP handbook for the US Nuclear Regulatory Commission.

 

Description of the tool

The THERP handbook presents methods, models and estimated human error probabilities (HEPs) to enable qualified analysts to make quantitative or qualitative assessments of occurrences of human errors in nuclear power plants (NPPs)

THERP is a total methodology for assessing human reliability that deals with task analyses (e.g. documentation reviews and walk/ talk through), error identification and representation, as well as the quantification of HEPs (Kirwan, 1994).

THERP is often referred to as a ‘decomposition’ approach in that its descriptions of task, have a higher degree of resolution than many other techniques. It is also a logical approach and one that puts a larger degree of emphasis on error recovery than most other techniques. Essentially, the THERP handbook presents tabled entries of HEPs that can be modified by the effects of plant specific Performance Shaping Factors (PSFs), using other tables (Swain, 1987).

The key elements to complete the quantification process are described by Kirwan et al (1997) as :

- Decomposition of tasks into elements

- Assignment of nominal HEPs to each element

- Determination of effects of PSF on each element

- Calculation of effects of dependence between tasks

- Modelling in an HRA event tree

- Quantification of total task HEP

To arrive at the overall failure probability, the exact failure equation involves summing probabilities of all failure paths in the event tree. When all the HEPs are .01 or smaller, the exact failure equation can be approximated by summing only the primary failure paths, ignoring all the success limbs. The

accuracy of the approximation decreases as the number of terms or the values of the HEPs increase (Swain & Guttmann, 1983).

 

Validation

Kirwan et al (1997) carried out an independent validation of THERP along with two other methods (HEART and JHEDI). They found that no one technique out performed the others, and all three achieved a reasonable level of accuracy.

The method has been extensively used in the nuclear industry, particularly in the USA.

 

Domain usage/ applicability to other domains

THERP was developed for probabilistic risk assessments of nuclear power plants but has been applied to other sectors such as offshore and medical.

 

Resources required to complete the assessment

Training on THERP is required before application. Specifically, Swain & Guttmann (1983) state that the method is intended to assist trained risk analysts in quantifying human reliability. Assessors can require one to two weeks of training to become accredited.

THERP can be resource intensive (Kirwan, 1994) but the available information does not give a clear indication about the average length of an assessment or how many people are required.

Swain (1987), when explaining the reasons for the abbreviated version of THERP, known as ASEP, the accident sequence evaluation program human reliability analysis procedure said that, “… the THERP handbook is thorough, for its fullest application it requires considerable manpower and time on the part of a team of experts, including a human reliability specialist, systems analysts, plant personnel and others”.

 

Availability of the tool & support

The THERP Handbook was produced for the US Nuclear Regulatory  Commission (NUREG/CR-1278) and is publicly available via their website.

There are a number of consultancies that provide training in human reliability assessments and may be able to provide training in THERP.


Pros and cons

The following is taken from Kirwan (1994).

Pros

- THERP is well used in practice

- It has a powerful methodology that can be audited

- It is founded on a database of information that is included in the THERP handbook.

Cons

 - THERP can be resource intensive and time consuming.

 

References

Kirwan, B. (1994). A guide to practical human reliability assessment. Taylor & Francis, London.

Kirwan, B. (1996). The validation of three human reliability quantification techniques, THERP, HEART and JHEDI: Part 1 technique descriptions and validation issues. Applied Ergonomics, 27, (6), 359-373

Kirwan, B., Kennedy, R., Taylor-Adams, S. and Lambert, B. (1997). The validation of three human reliability quantification techniques, THERP, HEART and JHEDI: Part II – results of validation exercise. Applied Ergonomics, 28 (1), 17-25.

Swain AD and Guttmann HE (1983). Handbook of human reliability analysis with emphasis on nuclear power plant applications. US Nuclear Regulatory Commission), Washington, DC. NUREG/CR-1278

Swain, A. D. (1987). Accident Sequence Evaluation Program Human Reliability Analysis Procedure, NUREG/CR-4772 (US Nuclear Regulatory Commission, Washington, DC .1987)


Summarized by : Manager of Mr . Safety web