A row in a periodic snapshot fact table summarizes many measurement events occurring over a standard period, such as a day, a week, or a month.  The grain is the period, not the individual transaction.  Periodic snapshot fact tables often contain many facts because any measurement event consistent with the fact table grain is permissible. These fact tables are uniformly dense in their foreign keys because even if no activity takes place during the period, a row is typically inserted in the fact table containing a zero or null for each fact.