aspdesigner - 7:02 am on Jun 12, 2010 (gmt 0)
@jdMorgan: Agree that it's better to do a re-direct, than to split your PageRank into two pieces.
"You can tell by the fact that Matt couldn't immediately answer Eric Enge's question that this is not a major issue"
@tedster: That's the impression I got as well. It was like it wasn't an intentional reduction, but more like when he asked a tech about it, they told him "yes, but because of how we implemented it..."
OK, I think I may have figured-out what they are doing here!
Consider this from a programmer's standpoint. If you're working on the code / database for calculating PageRank, how do handle a 301? You can't simply replace the old domain name with the new one - you need to keep the original URL around, so when you find new links pointing to that URL, you know where to pass the PageRank to!
The simplest/cleanest way, I would think, would be to simply consider the originating page as a node (page) for PageRank purposes, basically, storing the 301 like a normal page (original URL) with a single outbound link (destination).
If you do it that way, then the PageRank damping factor comes into play - that small amount of "juice" that gets subtracted from the total amount ANY given page has available to pass to its outbound links. This lost PageRank is small - in the original PageRank docs published by Larry Page, I believe the example given was 15%.
As the PageRank calculations are going to apply this damping factor automatically when calculating the outbound PageRank for each node, it would probably not be considered worth the extra time & trouble to write "special-case" code, just to exclude the damping factor from 301 pages.
So if that is the case, the loss would be no different then that naturally occurring on a regular page with an outbound link!
Matt gave us a clue that also appears to support that hypothesis, in the language that he used -
"...that sort of natural PageRank decay..."
Matt could have just as easily said that the loss is no different than that normally occurring when PageRank flows through any page, and we would instantly know exactly what he was talking about. But where is the fun & confusion in that? ;)
"On the other hand he did no specifically say that it only applies to domain name changes: it could well be that on site 301s lose some PR as well."
graeme_p, I would say that is most likely the case, for several reasons...
1) If this is just natural PageRank decay as a result of the way 301's are implemented (as I theorized above, and supported by Matt's wording), then there should be no difference where it is pointing to, the damping factor would be applied to the node in either case.
2) PageRank is calculated on a per-PAGE basis, not per site. PageRank flows through links from one page to another, regardless of location. Internal links pass PageRank, just like external links do. There is no reason to believe that Google would go to all the extra trouble to "special-case" 301's to calculate PageRank differently.
3) People seem to think there is something special about a domain 301. But technically, there is no difference between an individual page 301, and a site-wide domain 301. When you do a site-wide 301, the server actually has to generate individual page 301's for each page someone requests from the original domain. As far as Googlebot is concerned, there is no difference either - all Google sees are the individual page re-directs in either case.
4) PageRank calculations are one of the most computationally intensive tasks Google has to perform. It just does not make sense to slow that process down with extra "special case" coding, just to handle internal & external 301's differently.