I'm sure every company had rosters and roll calls, and that the first sergeants on up knew who had fallen out and when. I sure didn't mean to imply that the 15/16 Iowa was deficient in this regard.
But virtually no organization reports publically the following information:
*the number of people registered
*the number of people who showed up Friday
*the number present on Saturday
*the number present on Sunday
*all the above broken down by sub-unit and rank
Based on events where I've had access to this information it appears that there's a pre-event attrition rate that averages around 30%, but that rate is much lower for cavalry and artillery and higher for infantry privates. Attendance normally drops off from Saturday to Sunday, but not by much unless there's very bad weather or other factors at work -- like a hard march. At events in areas with many local reenactors, the number of walk-ons will generally cancel out the difference between registrants and those who actually show up.
Attendance seems to be higher for artillery and cavalry, perhaps because it requires a lot more work for them to get to an event in the first place and they don't commit casually. If they commit, they generally make it. "Campaign" events that demand more of their attendees seem to have the same effect. (At "Winter '64" virtually all attendees showed up, although it was a winter encampment northeast of Buffalo. Similarly, it looked the advertised strength of the 15/16 Iowa was largely present for duty Saturday morning.)
Are any of these conclusions valid? Do the answers vary from region to region? Have they changed over time? For different kinds of events, how many of the people registered can organizers count on actually showing up? How many will usually stay given the planned activities?
I think this information would prove pretty helpful. It shouldn't take that much work to pull together and post, but folks would have to commit to doing it beforehand and hang on to their registration numbers and morning reports.