Vamm - 5:01 am on Apr 27, 2010 (gmt 0)
If the users were running IE but NOT clicking on ads.
Then switched to FF+Adblock.
CTR goes UP because impressions stop.
If the users were running IE and clicking on ads.
Then switched to FF+ADblock.
CTR does not change because both impressions and clicks stop.
Unless all users switch to FF+Adblock, when impressions and clicks = 0.