Uncertainties due to imperfect knowledge of systematic effects: general   considerations and approximate formulae
  Starting from considerations about meaning and subsequent use of asymmetric uncertainty intervals of experimental results, we review the issue of uncertainty propagation. We show that, using a probabilistic approach (the so-called Bayesian approach), all sources of uncertainty can be included in a logically consistent way. Practical formulae for the first moments of the probability distribution are derived up to second-order approximations. 
G. D'Agostini, M. Raso