There is some debate over whether a sub-domain can pass seo power on to the main domain. Opinion is divided, but the most people seem to think that seo juice and therefore Page-rank, can empower the main domain of your website and with a good linking strategy, pass page-rank to all parts of a website.
That would make good sense, but then nobody seems to know for sure if Google classes a link from a sub-domain to a main domain as an external link.
External links have more weight with respect to search engines than internal links do. Naturally, well structured links with good anchor text in all internal links will help a tiny bit but, well structured in-coming links with well chosen anchor text help much more.
But how do the search engines class sub-domains.
This web design and seo website is a sub-domain of a personal blog about life on the Silver Coast. The personal blog has been left almost dormant for quite a few months and yet overnight it acquired PR2. The web design sub domain is being worked on and built up with articles and backlinks and at exactly the same time acquired the same PR overnight.
Is that a coincidence? It is difficult to argue with the facts here that seemingly Google upgraded a sub-domain due to the work being done on it, and as an extra reward it also upgraded the main domain. There has been no backlink work done with the main domain and no articles published since the middle of December, and that was the only one for several weeks.
This assumption inevitable leads to an anomaly which is that the web design for holiday homes sub-domain on the same site has acquired no PR.
There has been a fair amount of backlink work done on this sub-domain but the critical factor is possibly that the age of this particular sub-domain is only about eight weeks.
So what is going on here? Is the work on the PR2 sub-domain being mirrored by Google in the main domain and the other sub-domain being ignored for being to young? Or is it all just a coincidence? Does anybody actually know how Google accounts for things like this?
Another part of the equation is simply that perhaps the successful sub-domain has more and better quality backlinks at the moment than the other one.
It would be terrific to have the belief, that Google is rewarding the efforts to market the sub-domain, but the question of how long it should take to get ranking on the other sub-domain will hold another part of the answer.