Aida, It depends on the design of the course as well. Many people will say that entry level courses might have a higher ratio like 20 or 25:1 and upper level or graduate courses might have a 15:1. This is often correlated with the "design" of the course. For example, more discussions, and in-depth analysis of information would require more effort on both the student and instructor and therefore would ideally be a smaller ratio. These types of designs are often connected with upper level or graduate course content and therefore the correlation. Herbert Brown III