The introduction into geology of probabilistic models such as Markov chains for simulating stratigraphic sections, and independent-events models of drainage networks in geomorphology, calls attention to the need for re-examination of operational definitions used in conventional field and other kinds of measurement techniques. Critical examination of observed frequency distributions that arise from the measurements is especially important in terms of the physical processes that generate them. For example, some simulation models in stratigraphy give rise to geometrically distributed thicknesses of rock units. Unless the input is similarly distributed, the model does not reflect real-world events.
The quantification process involves at least five steps: (1) selection of qualities to be quantified; (2) development of operational definitions for obtaining the numbers; (3) evaluation of objectivity, accuracy, and precision of the resulting numbers; (4) examination of the frequency distributions of the numbers; and (5) critical evaluation of the usefulness of the numbers in the development and testing of geological models. Items (4) and (5) are emphasized in this paper, in terms of their bearing on the growing use of probabilistic models in geology.