just saw this on the adcom blog
"Adcom do not target any particular number of invitations per day, or even in total (invitations are issued on the basis of a quality, rather than a numerical, threshold) . Rather invitations are issued as evaluations are received back from readers, and processed by the back office staff - some days large numbers are received back and some days fewer. Adcom have a large pile of applications to work through and this takes time - some applications are amongst the first to be reviewed, and some are among the last, on a purely random basis.
However, it is fair to say that the nature of the review process is such that the vast majority of interview invitations go out towards the final couple of weeks of the interview 'window' rather than the first few weeks (indeed, typically a significant proportion go out in the final couple of days).
Ultimately trying to predict numbers and timing is pointless - both in terms of there not being a predictable pattern, and also in that it won't make YOUR invitation come any faster.
I would strongly advise going and enjoying a recreational beverage (or six) with some friends during this time, rather than tearing your hair out trying to work out exactly when your invitation will arrive."
easy for them to say to relax!!!!