Abstract
Random events may interfere with the execution process and results of the work. Then, the duration and cost of work are random variables. The random characteristics of these variables are described by using the β-PERT probability distribution and the simplified formulas. To determine the probable changes of the duration and cost of work, depending on the random factors, the coefficients of optimism and pessimism have been used. Additionally, the coefficients of improvement and deterioration of conditions of work implementation have been introduced. The values of these coefficients are calculated depending on the type, the probability of occurrence, the intensity of random perturbations and depending on the probable changes of the work duration as well as the probability and cost consequences of the price increase. Extreme random improvement and deterioration of the work execution conditions are analysed.