Robert - As you go through your professional life administering TSM, keep one thing in mind: the Summary table is unreliable. It has historically been unreliable. As testament, see the many postings from customers who have wasted their time trying to get reliable data from it.
As we keep saying: Use the TSM accounting records as your principal source of data for usage statistics. That's what they are there for, and they reliably contain solid, basic data about sessions. Beyond that, use the Activity Log, the Events table, and client backup logs. Richard Sims, BU