Combined Effect of Multiple Performance Shaping Factors on Human Reliability: Multiplicative or Additive?

Abstract

ABSTRACTWe addressed a fundamental question in human reliability analysis (HRA), namely: What is the functional form of the combined effect of multiple performance shaping factors (PSFs) on human unreliability? The combined effect of multiple PSFs on human error probability (HEP) is typically treated as the product of the effects of single PSFs on HEP in a multiplicative model or a sum of the effects of single PSFs in an additive model. However, there is a paucity of studies as to which model is more appropriate. In the present study, we collected data on empirical combined effects of multiple PSFs (198 data points for two PSFs, 61 for three PSFs, and three for four PSFs) from the human performance literature and calculated their multiplicative and additive effects. Both the calculated multiplicative and additive effects were positively correlated with the empirical combined effect. However, the median of the multiplicative effect exceeded that of the empirical combined effect while the median of the additive effect did not significantly differ from that of the empirical combined effect. Thus, the multiplicative model yielded conservative estimates while the additive model produced accurate estimates. Thus, the additive form might be more appropriate to model the joint effect of multiple PSFs on human reliability.