I predict, at P=.8 that there will be significantly more open thread posts in the second half of the month using this 2-post method.
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!
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.