Ask HN: How does position within SERPs affect click-through probability?

Suppose, for example, that the first search result on a page of Google search results is swapped with the second result. How much would this change the click-through probabilities of the two results? How much would its click-through probability drop if the fifth search result was swapped with the sixth?<p>Can we say something, with some level of assurance, about how expected click-through probabilities change if we do these types of pairwise swaps within pages of search results?

-- To clarify:

We're not after click-through rates relative to time (the sort of click-through rates which can be calculated from the leaked AOL date). What we seek are changes to click-through probabilities.

Click-through probabilities are determined by several factors, including (in no particular order):

(i) perceived relevance of the title; (ii) perceived relevance of the summary blurb; (iii) perceived relevance of the URL; (iv) position; (v) font/decoration.

What we'd like to know is: all things being equal (all the other determining factors), how does position affect click-through probability? If the fourth result, say, was switched with the fifth result on a page of search results, how would this effect their respective click-through probabilities?