I am not really following? I didn't mean that one of us could quantify this but rather G has numerous patents for forumlas that quantify variables such as link reputation to use within an algorithm. I just tried to offer some seemingly obvious variables they presumably use.
quantifying it may be another matter.
You may have to consider the up to a hundred ranking factors that G is supposed to use ;-)
The so-called evidence that first made me aware of the value of alt tags from linked images came from this G backlink checker I use often. It gives the link text from all links Google has on file for any given site.
Remove the alt, and it removes the link text.
The tool takes a while if you have dozens of links on file, but works fast if you have only a few so beware.
(This is not a G product and is only as good as the person who created it).
The other evidence in this thread seems pretty solid as well.