In the SEO realm there is a lot of attention paid to PageRank as an indicator of a domain's worth. Although it is probably one of the most important aspects of a domain's worth in the eyes of Google, there are a few things that should be mentioned.

To begin with, for anybody who doesn't know what PageRank is - it is a value used by Google to decide how important a domain is based on analysis of link on the internet. Google assigns each page on the internet a weight and when one page links to another it passes some score onto the page it is linking to. This is true for both external and internal links. Google PageRank has 11 scores, between 1 and 10. The higher the PageRank, the more likely Google is to trust it. Web pages that don't have any PageRank are often symbolised as having N/A PageRank.

There are those that suggest that the text content of a page affects PageRank assignment, but this doesn't seem to be the case. There is no substantiated evidence that text affects PageRank other than pages that have been spammed often have their PageRank removed.

When it come to sub-pages, time is a factor in PageRank distribution. When new sites first get assigned PageRank their sub-pages often remain without any PageRank. In general, sub-pages are somewhat slow to get give PageRank. Google is generally less trusting of sub-pages unless they belong to a trusted domain. In particular websites with a lot of sub-pages and even more so sites that link to a lot of internal pages (like directories) can struggle to pass their PageRank to internal pages.

Google can and do modify websites' PageRank. This is generally as a penalty for sites that have used unhonest methods. This can often cause all the site's sub-pages to loose their PageRank. It has also been claimed that in the past Google has made mistakes in PageRank distribution.

The structure of the site has a major role to play in the assignment of PageRank. Google uses what is known as block level analysis to pick apart web pages. They use their knowledge of the nature of the web to decide what links on a page are probably the most trustworthy and the pages these links point to are more likely to receive PageRank.

Probably the most important factor in the distribution of PageRank to a web page is the PageRank of the web pages linking to it. The higher the PageRank of web pages linking to a given page, the higher its PageRank is likely to be. PageRank is generally not assigned to a web page higher than the pages that link to it. So, if a web page has 4 PageRank 4 and 3 PageRank 3 links pointing at it, it will probably be assigned a PageRank of 4. In addition, web pages that have a lot of low PageRank and PageRank N/A hyperlinks pointing to them can have their PageRank reduced as a result. Sheer quantity of links does not help to increase PageRank. A site can have a high PageRank with relatively few links pointing to it if those links are of a high PageRank.

Lastly, the PageRank of a web page may change even when there has been no change in the hyperlinks pointing to that page. This is either due to changes in the structure of links between all websites or modifications made by Google.