Measurements of numbers, volume, and frequency of occurrence used traditionally in evaluating stomach contents of fish fall short of depicting true relative value. Numerous small organisms overshadow the importance of a few large ones. Differential digestive rates distort volumetric measurements. Frequency of occurrence tabulations are sensitive to sampling error. An ideal representative value would probably be one which integrates each of the above plus one for nutrition.
We developed an index of relative importance to assist in evaluating the relationship of the various food items found in stomachs knowing full well that it may fall short of some theoretical ideal. Our index of relative importance (IRI) was calculated by summing the numerical and volumetric percentage values and multiplying by the frequency of occurrence percentage value. Nutritional measurements, for example calories per unit volume, were beyond the project's capabilities.