Errors of omission have been accepted as an element of cost contingency in theconstruction industry. This paper presents mathematical models of contingency for errors of omissionbased on a variety of assumptions of probability distributions. A general model/equation fordetermining contingency is presented and then specific models are formulated by selectingappropriate probability distributions for the size of cost items and functions for the probability of theoccurrence of errors of omission. The proposed models are compared with the traditional percentagemethod. The assumptions underlying Model I are fairly simplistic and lead to the traditional percentagemethod being a perfect fit. Model II and III assume that smaller items are more likely to be omittedthan larger items. Given their respective assumptions the two models generate more accuratecontingency estimates than the traditional percentage method.
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