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the engineering cycle, whereas HRA is often used in the verification and validation phase, after systems have already been built. However, the application of HRA primarily to as-built systems is a historical artifact.

Analysts have included assessments of human reliability in military system evaluations since the 1960s (Swain, 1963), but the first widely publicly available guidance for HRA was described in the WASH-1400 report (U.S. Nuclear Regulatory Commission, 1975), which addresses the safety of nuclear power plants. The Technique for Human Error-Rate Prediction (THERP) HRA method (Swain and Guttman, 1983) provided the first systematic method of identifying, modeling, and quantifying human errors.

THERP and subsequent HRA methods developed in the aftermath of the Three Mile Island nuclear incident in the United States were accompanied by a call for risk-informed decision making using PRA and HRA (Kadak and Matsuo, 2007). Together, HRA and PRA produced assessments of existing systems with less emphasis on design than was typical with HFE and CE.

HUMAN RELIABILITY PROCESS MODEL

The three phases of contemporary HRA methods are depicted in Figure 1. As shown, HRAs can be characterized as qualitative or quantitative. A qualitative HRA includes the identification and modeling phases described below. It converges on other assessment approaches such as root-cause analysis, which is used to determine the causes of human errors. A subsequent quantitative HRA uses these qualitative insights to estimate the likelihood of these errors.

HRA Phase 1:
Identify the Sources of Errors

This phase typically consists of a task analysis to determine human actions and a review of those actions to identify opportunities for errors. Performance-

FIGURE 1 The three phases of HRA.

FIGURE 1 The three phases of HRA.



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