Inbound links have been both, the boon and the bane of search engines.

Even though linking still forms the basis of search rankings, it is also the most abused form of search engine spam. Consequently, Google has expended much effort to differentiate a natural linking process vis-a-vis an artificial one.

All inbound links to a document are judged on the following parameters:

• Appearance and disappearance dates of one or more links pointing to the document. A rapid rate of link churn could be an indication of a link farm and might adversely affect rankings.
• Freshness of links associated with the document. In most likelihood freshness has a positive impact only when the document containing the links is trusted. Freshness in turn is a function of the following parameters:
  • Date of appearance/change of the link
  • Date of appearance/change of anchor text associated with the link
  • Date of appearance/change of a linking document containing the link.
• Determining how much a document containing the link is trusted, how authoritative a document containing the link is, and a freshness of a document containing the link. This is points to the pretty known fact amongst the SEO community that links from related and high Page Rank sites help increase rankings more than links from unrelated sites which also have a poor Page Rank.
• Determining an age of each link (and the corresponding linkage data) pointing to the document. The ranking is an inverse function of the age of the linkage data. The linkage data includes a links and anchor text provided by one or more linking documents and related to the linked document.
• The rankings are also positively correlated with the longevity of the linkage data and an indication of content update of the linking document. A relevant and regularly updated page linking to the document over a long period of time, further corroborates the relevance of the document. Note that the patent application also specifies a negative correlation in case the longevity indicates a short life. In other words, an inbound link is most valuable for search rankings in case it comes from a relevant and regularly updated page and has retained its position on the page for a long period of time, in spite of the regular updates.
• Determining an indication of link churn for a linking document providing the linkage data. The indication of link churn is computed as a function of an extent to which one or more links provided by the linking document change over time. Search rankings are penalized if the link churn is above a threshold, which again points to the existence of a link farm.

Most of the above are measures taken to check link spam. The lesson to take home is that Google has gone to great lengths to prevent link spam, and is most likely going to keep evolving its algorithm further in this regard.

Hence, webmasters would do well to focus their attention on providing good and relevant content which abets a natural linking process, instead of searching for sources of easy links, which might after all have a negative affect in their rankings.