There is a guy that puts the lists into Excel, runs a sort on duplicates and removes them, and you are left with all of the names that appear on one list and not on the next. Shows all of the people that left the list. Pretty easy way to tell the "real" attrition. But then you can go in and look at the people that are already gone and still on PER leave, and others that left over a month ago and are still on the list as active, and you can see that the real number is actually a little higher.