If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
If continuing the discussion becomes impractical, that means you win at open threads; a celebratory top-level post on the topic is traditional.
Poster's Note: omg, it felt so weird typing "2012" up there.
Why would you test just the second half of the month, versus total posts in a month?
It seems that you would actually want to measure total participation in a month. I see no reason not to measure it directly, rather than indirectly.
Good idea! Thanks. It requires more counting, but I think it's the better way. Experiment updated!