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in function ss(), the one that displays "go to www.example.com" there is now support for a parameter (nm=) in the URL that is incremented everytime a user does a mouseover on the ad's link.
So, you start with nm=1 and go on nm=2 , nm=3 etc.
My guess is that the distribution of the nm values is useful in click auditing and fraud checks. Generally speaking, a good site will have higher values than a poor site
In a good site the user looks around for a while before clicking.
On the other hand, if all the clicks from a site have nm=1 then something is very fishy, people go there just to click on ads.
Google could also check this data with other info we don't see (e.g. the amount of time that has elapsed between the ad being served to the user and the time that the user has clicked on it) to profile the intention and quality of the clicks a site generates.