Originally posted by KazetNagorra
You would expect to have a slight majority of threads with an odd number of posts because you start with an odd number (1).
And a lot of threads die with only 1 post. But it's not enough to explain the preponderance of threads with an odd number of posts, because the preponderance remains even if the one-post threads are not counted. I think much of the rest of the phenomenon can be explained by threads that "almost" die but eke one response from someone to whom the thread's creator almost invariably replies. This results in a fair number of threads that die at 3, and also (less so) at 5.
Speaking of funny patterns, at this writing there are three threads on the first page of the debate forum that have exactly 20 posts, and not only that, they're all clustered together!