A 2009 Journal of Law and Economics article by Marc T. Law and Mindy S. Marks suggests that during the period 1890-1950 occupational licensing did not tend to affect blacks and women adversely. The biggest problem with the paper is that a Census-reported practitioner in a licensing state is not necessarily licensed—a fact never mentioned by Law and Marks—and yet that fact should greatly affect their treatment of the data and results. Information about plumbers in Maryland reveals that in treating the Census number of black plumbers as licensed black plumbers—as Law and Marks implicitly do—they overstate the actual number by 4700 percent. It is therefore unsurprising that they do not find plumbing licensing to have negatively impacted blacks. The paper suffers from several other problems, as well, including: Law and Marks lump certification in with licensing; there is a sample selection bias in their method for including an occupation in the study; several of their findings are based on extremely low participation by blacks and women; they treat women as the “minority” in the field of nursing. Because of these and other problems, including the results of falsification tests, we judge their conclusions to be highly doubtful.