The author's previously developed theory on income distribution, in which two of the explanatory variables are the average level and the distribution of education, is refined and tested on data selected and processed by the author and data from three studies by Americans. The material consists of data on subdivisions of three countries, the United States, Canada, and the Netherlands. The main difference between the author's theory and those of the others is the introduction of demand by the "organizers of production" for skill or qualification alongside with supply. One of the points of focus of this essay consists of attempts to give practical shape to the introduction of variables supposed to represent demand. The theoretical base is advanced in the first section, followed by a discussion of some characteristics of the testing material used. In the remaining sections of the essay some results obtained for the three countries mentioned are shown and compared with results obtained by others. (Author/MLF)