While human error has long been recognized as a contributor to process incidents, Process Hazard Analysis (PHA) teams often struggle to agree on the frequency at which human errors might occur. This difficulty in reaching a consensus can be due to the varied backgrounds and experiences of team members, as well as the degree to which they believe human error is inevitable as opposed to simply being a reason for discipline. A process that combines research-based, quantitative estimates of human error rates with plant experience and intuition is proposed for use during qualitative PHAs. In this process, a Base Error Rate is selected, from published sources, for the error under consideration. The PHA team then assigns Error Rate Modifiers to reflect scenario-specific conditions that may impact the Base Error Rate. Site-specific conditions may include Time Pressure, Operator Experience, Procedure Adequacy and Fatigue, although others may be used. Each Error Rate Modifier can increase or decrease the Base Error Rate by up to one order of magnitude. The Base Error Rate is multiplied successively by each of the Error Rate Modifiers to obtain the Adjusted Error Rate, which is more reflective of actual plant conditions than the generic Base Error Rate. Use of these quantitative estimates within a qualitative Process Hazard Analysis is described, along with validation data for the technique, and one facility's successful implementation of this process.

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