The effectiveness of safety countermeasures is often estimated from "before" and "after" accident histories of systems on which the countermeasure has been implemented. Due to practical reasons, systems are at times selected for treatment on the basis of their poor safety performance. This process of selection will ensure that on the average a reduction in the number of accidents will be observed even if the countermeasure has no effect. In this paper a robust and simple method for the estimation of this bias-by-selection is obtained. It allows elimination of the bias from research results. The dependence of the bias-by-selection on the duration of the accident history available and on the severity of the selection criterion, is explored. (Author/publisher)
Abstract