A probability model, also known as a stochastic model, is a mathematical formulation that incorporates an element of randomness. This contrasts with a deterministic model, in which reliable predictions are made on the basis of observed variables. The simplest probability model is the Gaussian, or normal, distribution, of which there are many examples in biology, medicine, and public health. Variations in height, weight, blood pressure, and attack rates in outbreaks of disease are examples of Gaussian distribution.
There is an element of randomness in the distribution of many disease states, although deterministic factors also play a part. Thus, it is possible to estimate risks—such as the risk of cancer following exposure to a known carcinogen— but not to predict which individuals among the members of a high-risk group will develop cancer, or when. Many stochastic models display variation only within a relatively narrow range, however, so prediction, and therefore planning, is feasible within these limits.
JOHN M. LAST