Backlinks

The number of links to a Web page or Web site and their quality is used by search engines as a major determinant of the authority of a Web page or website. Increasing backlinks is the major part of so coalled Off Page Optimization. The Google PageRankä algorithm is the only public explanation of the use of backlinks by search engines. It is only a partial explanation.

The public part of the algorithm explains that authority of a page is determined as a cascading function of the authority of the pages linked to that page. Each page is assigned a PageRank number based on the number of pages links to it, their authority and the number of links on those pages. A given page thus has a given amount of "link juice" that it can distribute to other pages. If it has more outgoing links, each of the linked pages derive less authority. Thus, if Page P gets links from pages a,b,c of authority A, and those are the only links from pages a,b and c, its rank will be higher than that of page P' that gets links from pages d,e and f of the same authority, which each send several links to different pages.

This algorithm is used by Google to determine the general authority of a page. However it is probable that the value of a link in the algorithm as implemented is not only a function of  the PageRank divided by the number of links on a page. External links are probably treated differently from internal ones, and one suspects that a constant is added that reflects the authority of the originating page, so that the value of links from high authority page is not watered down is proportion to the number of links from the page.

Repeated links from the same website are probably not as valuable as links that originate from several different sources.

PageRank and authority do not directly determine the positioning of a page in search engine results for a given  keyword. The anchor text of the links does have a direct influence on the positioning in Google and probably in other search engines. For example, if a page gets numerous links with the word "widgets" in the link, that page is likely to list high for keyword "widgets." If additional links are added with keyword "plumbago," the page will be be listed in higher positions for keyword "plumbago" as well. This seems to be true whether or not the target page in question mentions the keyword "plumbago."

This feature was used by spammers and pranksters for "Google Bombing" - linking to a page with thousands of links that use an irrelevant or politically loaded phrase, so that that page would display at the top of search engine results for the phrase in question. For example, Google "Complete Failure" and web page with a picture of George Bush would be listed at the top of the results.  Google is said to have taken specific precautions against this practice, but it is still possible to boost the ratings of a page by targeting it with links using a specific phrase or word.  

The precise way in which backlinks influence the position of a page or Web site for a particular&in search engine results is known only approximately. 

Viewing Backlink Information

If you type link:mysite.com then Google will list some of the backlinks for mysite.com, but not all of them. The listing does not differentiate between same-site links and off -site links. Various free tools are offered on the Web for determining the number of  backlinks, but most do not appear to work. This one http://www.backlinkwatch.com/ appears to work at least partially, and provides the number of links on the originating page as well as other information such as pagerank and whether or not the link has a "nofollow" attribute. It is very slow however, and when I ran it, a script timed out before it could list all the links for a large site and provide all the information. Yahoo! currently offers several services for determining inbound links to a site.

In the regular Yahoo! search type:

linkdomain:yourdomain.com -domain:yourdomain.com

to get the links from other Web sites only to yourdomain.com,

or

linkdomain:yourdomain.com

If you want to see all the links including those from yourdomain.com. There is also a Yahoo! site explorer API tool for this purpose.

Google shows some of the backlinks for a site (eg yoursite.com)  if you type

link:yoursite.com

The number of links displayed is only an arbitrary sample. Google Webmaster tools displays more detailed information, though not necessarily accurate information.

The results for the same domain are very different from different sources:

Google backlinks:     1,610

Google Webmaster:  168,541 external+67,454 internal

Yahoo! backlinks:      93,103

Backlinks.com:           32,738 

While Google Webmaster found the most links, it claimed that the main page of this site gets only 710 internal links. The site has over 2,000 pages, each of which link to the main page, and all of which are registered in Google. The listings may also include bogus duplicate variants that search engines create such as links from http://seo.yu-hu.com and links from http://www.seo.yu-hu.com.

Quality of Backlinks

It is always better to get links from higher ranking Web pages  and legitimate directories rather than from lower ranking ones. There is no point in getting links from linkfarms that are set up up for the sole purpose of selling links. It is not known if backlinks from such "bad neighborhoods" can hurt your website.

Reciprocal Backlinks

There are various superstitions about reciprocal backlinks or link exchanges that are probably all untrue:

Never exchange links with a lower ranking website - this is certainly untrue, as the lower ranking site, if it is legitimate, will probably grow. Any link you get helps you. If the site is not direct competition there is certainly no reason not to exchange links.

Reciprocal links count less than one-way links - there is no reason to assume that this is so, and it probably takes too much computation time to determine if a large site has a link on any page to another large site.

Don't link to a site if they don't link to you - Obviously you do not want to help direct competition, but linking to an authoritative site can help improve the authority of your page for the relevant keywords. 

Linkspam and Backlinks

Unethical SEO firms and website owners sometimes abuse Weblog comments and other interactive tools to leave links to their unrelated websites, which may be exploitation sites of various sorts or political websites. Different methods are used to control linkspam, such as disallowing links in comments, or using the "nofollow" tag.

Nofollow tag for Backlinks

Google instituted the tag rel = "nofollow" for backlinks to be used primarily from the comment section of Weblogs, as a way to control the activity of spammers. Others, especially Wikipedia are using this tag as a way to control linkspam or as a  malicious way to prevent giving credit to authoritative sources.

Ami Isseroff

October 1, 2008

Note - Definitions of Search Engine Optimization terms are based on inferences from common usage and definitions given by other sources. Conclusions about search engine behavior are based on understanding of the behavior of the most popular search engines. Both are subject to error or may change. Search engine company management may define or use a term or set or change any policy in any way they see fit, and may make these definitions and specifications public or not. These decisions and definitions are beyond our control.  

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