Probability is a mathematical concept that allows predictions to be made in the face of uncertainty. The probabilistic approach in this page defines two types of uncertainty that are associated with small-scale inherent variability, commonly is associated with relatively small (meters-length) scales. The two types of uncertainty associated with small-scale inherent variability discussed are Measurement Error and Small-scale Geologic Variability. For both types of uncertainty, it is assumed that there is an underlying population that exactly defines the system of interest. As examples of small-scale inherent variability, the proportion of ripple drift lamination (geologic variability) at any location is a fixed constant, whereas the proportion within the reservoir is those constant values integrated over the entire reservoir volume.