RhinoFish - 8:23 pm on Dec 3, 2011 (gmt 0)
since the CPC costs vary with position, i say the real question isn't CR alone, but CPA or ROAS.
often, if you're adjusting bids for ROAS / CPA, your lower converting stuff will be bid down intentionally, making it appear that a lower position corresponds to a lower CR, but it's likely a lower CR corresponds to a lower position (because you put it there).
your site will be a bigger determiner of CR than your position (for the same ad).
further, the other things you can segment, will, in many more cases than the position case, be a bigger determiner of your CR, and also your CPA / ROAS.