The definition of churn rate was the subject of a shareholders' lawsuit against Netflix in 2004. The shareholders accused Netflix of reporting an artificially low churn rate using:
Number of Customers churn / (Number of customers at beginning + number of customers gained).
Where the plaintiffs preferred:
Number of Customers churn / (Number of customers at beginning + number of customers at end of period)/2.
Netflix succeeded in having the suit dismissed, since there is no official way to calculate churn.
This has happened to me before in full web pages where there is no clear visual indication of the end of the page. This and makes me question the scrollbar decision from a usability perspective - there is no direct visual signal of progress.
That's very interesting context and all the more reason to spend time thinking about this stuff (or face the litigious consequences). I hadn't heard of this case before writing this article but I don't find the similar definitions all that surprising. We tried to stay pretty close to the sorts of definitions one might learn in an intro financial analysis course.
http://www.globenewswire.com/newsroom/news.html?d=62086
http://www.docstoc.com/docs/33875708/In-Re-Netflix-Inc-Secur...