"How to do it" footnote

A while ago I wrote about interpreting ticket-sale statistics, as what I hoped would be a helpful guide for journalists trying to make sense of those numbers. I had a lot to say about subscription sales, since these are especially tricky. When someone says "our subscriptions are up," do they mean the number of subscribers, the number of tickets sold to subscribers, the amount of money made from subscription sales, or the percentage of total sales that subscriptions make up?

So here's something else to think about, most helpfully offered in an e-mail by David Wyeth, the Director of Marketing & Visitor Services at Carnegie Hall. Of course I'm quoting it with his permission:

One other key element is renewal numbers. While selling new acquisition subscription tickets can sometimes be nearly as expensive as single ticket marketing, renewals are the cheapest of all to sell. Plus it gets into the whole question of "churn," something a lot of us are dealing with these days. You may have terrific overall subscription numbers, but if it's mostly new people year after year, then it's costing you too much and it doesn't bode well for your near future (i.e.when do you run out of new people?).

Thanks, David. Everything I know about things like this, I've learned from people like you.