The advantages and limitations of using a back analysis of slope failure to evaluate soil shear strength are discussed. A methodology is presented herein that allows the implied level of reliability associated with soil shear strength parameters back calculated from slope failures to be estimated. A reliability approach is also used to estimate the probability of failure for a given limit equilibrium slope stability method design factor of safety and combination of back calculated Mohr-Coulomb shear strength parameters c', φ'. The methodology is illustrated using 39 landslides in the Orinda Formation in the San Francisco Bay area.