Editors' Note: Admissions officers often proudly publicize an institution's “average class size,” leaving the teaching staff wondering “Why don't I ever get any classes like that?” Here we learn why.

Professor Hemenway is a Professor in the Department of Health Policy and Management at Harvard University. An economist by training, he is currently director of the Harvard Injury Control Research Center and the Harvard Youth Violence Prevention Center. He has written extensively on firearms injuries and the economics of health care.

Most schools advertise their “average class size,” yet most students find themselves in larger classes most of the time. Here is a typical example.

In the first quarter of the 1980–81 academic year, 111 courses including tutorials, were given at Harvard School of Public Health. These ranged in size from one student to 229. The average class size, from the administration's and professors' perspective, was 14.5. The expected class size for a typical student was over 78! This huge discrepancy was due to the existence of a few very large classes. Indeed, only three courses had more than 78 students. One enrolled 105, another 171, and there were 229 in Epidemiology.

Given one class of the size of Epidemiology, an expected class size of approximately 78 for a typical student can be achieved in various ways. Four possible configurations for the rest of the classes are: (i) 450 individual tutorials, (ii) 50 courses of size 10, (iii) 25 courses of size 30, (iv) 25 courses of size 50.