It's difficult to say without looking at your workbook - if you can post a sample, that would help.
Just off the top of my head, I'm thinking that you might need another table in your data that is similar to a calendar table, but, in this case, would have a row for each individual 15 minute time point. And then for each of these time points, you can check to see if a class is in session or not during that time.
Thanks for your quick reply!
Here is a link to the workbook.
The graph shows the number of classes that start at a specific time. I want to know how many are in session at any given point in time.
I don't know if this is going to work for you or not. It's not a very elegant solution. What I did was create two new tables: (1) the first is the time table, that has a single row for each 15-minute-increment time point, and (2) the second is a table that contains a single row for each class and each time point. This second row contains a little over 41,000 rows. Then joining these tables along with your data table, I can create a new calculated field called "IsClassInSessionAtThisTime", that looks like this:
And that measure can then be summed at each time point to determine the number of classes in session at any given time point.
I'm attaching a sample workbook for you to examine. Like I said, this is not a very elegant solution, but it might get to what you are trying to do.
Hope it's helpful. Please let us know if thsi meets your needs.
p.s., I just noticed that I did the 15-minute increments incorrectly - sorry about that. The concept is still the same, so I hope you can get the idea from this example, and then implement it correctly if it's what you need.