By merging elasto-plastic finite element analysis with random field theory, an investigation has been performed into the stability of an undrained clay slope with spatially varying shear strength. The results of the study lead to a direct comparison between the traditional Factor of Safety and the probability of Failure for a range of statistically defined input shear strength properties. The results highlight the influence of the spatial correlation length, a variable routinely omitted from conventional probabilistic studies in geotechnics.