In the Preface to the First Edition of American Men of Science [ams1933jc] Cattell states that:

"A star is prefixed to the subject .. in the case of about a thousand of the biographical notes. These are the thousand students of the natural and exact sciences in the United States whose work is supposed to be the most important. In each of the twelve principal sciences the names were arranged in the order of merit by ten leading students of the science. the average positions and the probable errors were then calculated, so that in each science the order of merit was determined together with its validity. The names were then combined in one list by interpolation, the numbers in each science being taken approximately proportional to the total number of workers in that science. the thousand are distributed among the sciences as follows: chemistry, 175; physics 150; zoology 150; botany 100; geology, 100; mathematics, 80; pathology, 60; astronomy, 50; psychology, 50; physiology, 40; anatomy, 12; anthropology, 20. The star means that the subject of the biographical sketch is probably among the leading thousand students of science of the United States; but its absence does not necessarily mean that the subject of the sketch does not belong in this group, as the name may not have been considered in making the arrangements."