In pursuing this analysis, Galton's first problem was to develop a method for assessing high levels of mental ability. The approach that he took was to assume that mental ability is closely correlated with eminence (reputation) in a given profession. As he described it, "I feel convinced that no man can achieve a very high reputation without being gifted with very high abilities; and "few who possess these very high abilities can fail in achieving eminence."
Galton then attempted to marshal evidence in favor of the proposition that mental ability is inherited. First he examined the shape of the distribution of mental ability. Analyzing the scores of 200 candidates who had taken the Mathematical Tripos at Cambridge as well as those that had been obtained by 72 candidates for civil service positions, Galton showed that these scores (and hence presumably the psychological characteristics underlying the scores) were distributed in much the same way as inheritable physical traits, that is to say, normally.
While this similarity in the shape of the distribution of mental and physical characteristics did not in itself imply the inheritability of mental traits, it was consistent with Galton's claim. More importantly, it also allowed him to estimate the percentages of men that would be expected at each of a series of "levels" of mental ability ranging from the highest to the lowest. This, in turn, provided a standard against which the hypothesis of inheritability of mental ability could be evaluated.
