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Judging by a patent application filed by Google for ranking blog search, one of the three things a blog should have is a blogroll, and being included in some high-quality ones will help too.

Google Loves A Tasty Blogroll

SEO by the Sea blogger Bill Slawski has made a practice of diving into the rough waters of patent language, and emerging with pearls of detailed, readable wisdom.

His recent blog post about a filing by Google for ranking blog search turned up several points that could help a blog show up in a search, or keep it from being a factor.

Slawski found that Google considers blogs to contain three content types:

• The content of recent posts,
• A blogroll, and;
• Blog metadata (author profile information and/or other information about the blog or its author).

While it's likely Google is going to look at other factors in ranking blogs, the positives and negatives observed by Slawski show how the process resembles the fabled PageRank Google has used as part of its website rankings.

Positive factors for a blog include its popularity, inclusion in blogrolls (especially high-quality ones), and tagging of the blog. News aggregator subscriptions could imply high popularity. Google said that it could validate those against "subscriptions spam" by checking for unique subscriber IP addresses.

On the positive side of the calculation, Slawski suggested how Google might calculate another factor mentioned in the filing:

Implied popularity

Instead of explicit subscriptions, an “implied popularity” could be calculated from data collected from people searching on Blog Search, and examining the click stream of search results:

For example, if a certain blog document is clicked more than other blog documents when the blog document appears in result sets, this may be an indication that the blog document is popular and, thus, a positive indicator of the quality of the blog document.

Some negative factors could impact a blog. New posting frequency, especially at set intervals, could be a tipoff of a low quality, i.e. spam, blog. Post content and size might raise alarms in Google's algorithm, and push the blog to a lower ranking.

Slawski offered a couple of points where such negative ranking could affect a blog that actually contains valid content. A blog about Nigerian spam emails, he noted, may not rank well by Google's criteria.

Google could also divine some unsavory intent by the way links get sprinkled throughout a blog, by Slawski's reckoning:

Link distribution of the blog document

It appears that under this quality scoring system, whom you are linking to is considered, too:

As disclosed above, some posts are created to increase the pagerank of a particular blog document. In some cases, a high percentage of all links from the posts or from the blog document all point to ether a single web page, or to a single external site. If the number of links to any single external site exceeds a threshold, this can be a negative indication of quality of the blog document.

At the end of January, Robert Scoble compared a handful of blog searches, including Google's. Even though he quibbled with Google's default presentation of relevance sorting, he liked it far more than Technorati.

Going by Slawski's findings, that's because Google has a lot going on in the engine room of its blog search service.