Pamela ,
I agree with all of your statements. The optimal number does vary based on content, level, etc. which are also tied to the criteria and factors you mention. Since most courses have some level of student to student and student to instructor discussion and interaction, many people see 15-20 as an optimal number for courses with some level of interaction. The experience of the instructor can play a role; however, for a class with high interaction there are only enough hours in a day that can be used to respond to posts, etc. so there would still be… >>>