Three probabilistic methods of different complexity for slope stability calculations are in the paper evaluated with respect to a well-documented case study of slope failure in Lodalen, Norway. A finite element method considering spatial random fields of uncorrelated parameters c (cohesion) and u (friction angle) is taken as a reference for comparison with two simpler methods based on Taylor series expansion, known as first-order-second-moment (FOSM) methods. It is shown that the FOSM method enhanced by a reduction of variance of input parameters due to spatial averaging along the potential failure surface (extended FOSM method) leads to a significant improvement in predictions as compared to the basic FOSM method. This method is computationally inexpensive and can be used in combination with any existing finite element code, it is thus a useful approximate probabilistic method for geotechnical practice. Several limitations of the extended FOSM method for calculating probability of a slope failure are
identified.