This study investigates how estimates of uranium endowment made by a geologist using an appraisal system that is based upon a formalization of geoscience and decision rules compare with estimates made by informal and unconstrained intuitive processes. The motivation for this study derives from the premise that formalization of decisions would mitigate the heuristic biases and hedging that may result from the use of unconstrained intuitive processes.Estimates of the uranium endowment of the San Juan Basin of New Mexico by four methodologies are compared in this study. These methods, ranked by degree of decomposition (mitigating of heuristic bias) and control on hedging, are as follows: implicit 2, 1.5 X 10 6 a short tons of U 3 O 8 ; implicit 1, 1.6 X 10 6 short tons of U 3 O 3 ; NURE (1980), 2.4 X 10 6 short tons of U 3 O 8 ; and appraisal system, 3.9 X 10 6 short tons of U 3 O 8 . The magnitude of expected uranium endowment estimated by these methods, ranked from smallest to largest, is in this same order. With the exception of the NURE estimates, the magnitude of the variance (uncertainty) of uranium endowment, ranked from smallest to largest, also is in this same order. These results prompt the suggestion that the more decomposed and formalized the estimation procedure, the greater the expected value and the variance of uranium endowment.Equivalently, predicating U 3 O 8 endowment estimation strictly upon that part of the geologist's geoscience that is useful in making 3 O 8 endowment estimates and upon his understanding of the region's geologic history produced larger estimates than have previously been reported. However, this method of estimation also shows that uncertainty about the actual state of U 3 O 8 endowment is much greater than previously described.