To add to tedster's commments - I still firmly believe in trustrank with google. I agree with everything tedster says about the page and link being worthless under the wrong circumstances. But I believe that if you get an exact like-for-like link from a .gov and a .biz (i.e. identical pagerank, backlinks, topic, anchor text, number of outgoing/internal links etc. etc. etc. etc. etc.) there is a high probability that the .gov is worth more.
That kind of
contradicts this part of Ted's post:
It's the strength of the page itself that gives value to any backlink, not the fact that the TLD is .edu or .gov.
Which means it can be a .biz and still be better than a .gov based on the link strength, on page factors, etc.
First off, in my opinion trust rank is mostly a phrase that has been wrapped around a group of observed effects. I use the phrase Trust in that sense, the sense that there are various methods of determining whether a site is good or no-good. Here's a video of Matt Cutts
[youtube.com] talking about Trust Rank from three years ago, explaining Trust Rank.
I think what you are getting at is part of the algorithm that tries to predict whether or not a web page is likely to be spam or not spam, to answer the question or not answer the question. As far as I can recall, the first time I heard Googler's discussing statistical probabilities of sites being spam, was back in 2005 at PubCon New Orleans. They were talking about creating statistical models of what is spam and using that data for keeping the SERPs clean, that kind of thing. That's when I began noticing that some of Googlers didn't have degrees in computer programming but in areas related to statistics.
So maybe what you are getting at is more related to whether a .biz site is statistically more likely to be spam over a .gov. It's not an unreasonable assumption. It's not unreasonable to assume there might be a little more scrutiny for a .biz, but then again every site is scrutinized. But what is gained by scrubbing a .biz twice while potentially letting a .com slip by because it was underscrutinized?
Bandwidth? Computing cycles?
Some people may point to the SERPs and rightly note that there are very few .biz sites ranking. But is it because .biz domains can't rank? Or could it be because most people would prefer to think of another domain, one that's a .com, even it has a hyphen or two in it, than invest in a .biz that is perceived as risky?
It's like the lure color debate with fisherman. One fisherman may swear by a firetiger over a baby bass coloree lure and virtually always use the firetiger over the baby bass. Yet the act of using firetiger more often skews the results. If he used the baby bass during the summer when the babies were swimming around, he actually might have caught more fish using that lure. But it becomes a decided outcome through the choice he made. (And of course there's always the grizzled old timer who will claim it doesn't matter what color the lure is, it's all about how it wiggles in the water. :o)
Getting back to the topic, there are a lot of .edu pages that are useless for ranking. Some are link farms with barely three inbound links to their credit. In general, a .edu or .gov is likely to not have backlinks from SEO sites, wordpress widgets, and articles copied from article directories. So in that sense, yes they are better than the average dreck that is out there.