The comprehensive-versus-selective school issue is primarily a socio-politico-economic rather than pedagogic problem. The International Project for the Evaluation of Educational Achievement (IEA) had as its primary objective to relate certain social, economic and pedagogic characteristics of the different systems to the outcomes of instruction in terms of student achievement and attitudes. We limit ourselves here to problems related to the different systems and focus on the problem given in the slogan "Does more mean worse?" The IEA mathematics survey comprised of 12 countries and random samples of about 133,000 pupils from 5,450 schools. Four target populations at the 13 year and pre-university level were sampled and tested in all the countries. The Science survey comprised of 258,000 students from 9,700 schools in 19 countries. The students were sampled by random from four large populations. (1) Students aged 10:0-10:11, when they, in all countries, still were taught by one teacher in a self-contained classroom. (2) Students aged 14:0-14:11. (3) Students in the last grade of the compulsory school. (4) Students in the grade from which transfer to the university occurs. The mathematics investigations revealed a sharply fluctuating average level between countries among students in the senior, pre-university class. However, these comparisons of arithmetic means were not especially interesting unless allowance was also made for variations between countries in their recruitment bases or "retentivity." If that was done and equal proportions of cohorts were compared, the variations turned out to be considerably less on an average. (Author/JM)