Why student attendance is not considered a fact in data warehousing? In a KimballGroup example student attendance is not considered a fact. Hence it considers this example as forming factless fact table. I don't really understand what is the difference between a fact and measure. Why a sale, which is also an event, is considered a fact but the event of attendance is not. What is the difference with this two events? 

thanks
Blame Kimball for the confusing term. Attendance is a fact (as in, "a thing that is known or proved to be true"). However, it is an event, and as such it does not have any extra information apart from references to the corresponding dimension keys. It probably should be called "measureless fact". 

A sale, in addition to the dimension references, has measures: quantity and price.