Pitfalls in risk calculations

Two pitfalls in risk and reliability calculations are identified. The first is the treatment of the failure rates of nominally identical items as independent variables, when the states of knowledge that determine their distributions are identical. Such analyses result in underestimation of the uncertainties. The second is due to the use of the linear approximation to the exponential distribution or the use of lognormal distributions for frequencies of failure per demand, which results in erroneous means and variances.