A methodology is presented for selecting input strength parameters for deterministic c-ϕ slope stability analyses to account for spatial variability and cross-correlation of the material properties. It can be non-conservative to use mean values of the properties in slope stability analysis since the critical failure path will preferentially develop through the weakest materials. For materials exhibiting spatial variability, the average strength along the critical surface will be less than the mean strength, so the mean factor of safety from a probabilistic analysis will be less than the deterministic value. Recent publications have emphasized the importance of considering spatial variability and cross-correlation to account for these effects. However, not every project has sufficient data or means to conduct rigorous probabilistic analyses. The proposed method adjusts the c-ϕ parameters to account for spatial variability and cross-correlation. The result is a deterministic factor of safety matching the mean value from a probabilistic analysis