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Saturday, August 9, 2008

About Inbound External Links

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Today, a colleague asked me what I know about external links. Rather than simply e-mail her back I decided to sit down and answer this like I might if I were taking a college course exam. In other words a time limited brain dump. Let’s see how much I remember. :)

A Little Foundation

  •  InLink = Inbound Link - A link from another page or document on the Web to the page being examined.
  •  OutLink = Outbound Link - A link from the page being examined to another page or document on the Web.
  •  Internal Links - Links between pages or documents within the same domain.
  •  External Links - Links between different domains.
  •  If a web page links to another web page more than once it is only counted as one link.

Most often when we discuss links we mean external links.

Two Reasons Links Are Important

  1.  Links indicate trust and authority
    •  The more pages that link to a web page, the more trusted that web page becomes, especially when those links are from other web sites. (Quantity)
    •  The more search engines trust pages that link to a web page, the more trusted that web page becomes, especially when those links are from other web sites. (Quality)
    •  The more domains that link to a domain, the more search engines will trust the domain receiving those links. (Quantity)
    •  The more search engines trust domains that link to a domain, the more search engines will trust the domain receiving those links. (Quality)
    •  The longer a link exists the more trusted it becomes. (Quality)

A page receiving 1,000 low quality links may receive the same amount of trust from the search engines as a page receiving 10 high quality links.

  1.  Links provide context and relevance. - Words in links matter. If 1,000 pages link to a page with the anchor text or link text, “iPod listening station reviews,” the search engines will thing that the page being linked to is relevant for iPod listening station reviews. (The search engines will want to confirm this by checking the contents of the page, but in the past, pages routinely ranked for search engine queries where the only place the query text existed was in links from other web sites.)

Link profiles are among the most influential ranking factors measured by search engines. In competitive search query spaces, the web sites with the best link profiles are most often the top ranking web sites.

Natural Link Acquisition

Link acquisition is vital to effective search engine optimization.

The rate of link acquisition is an important part of link building. Search engines abhor manipulation. An unnatural pattern of link acquisition will appear manipulative. Falling link acquisition can signal decline.


  1.  Natural Link Growth - A steady rising and falling pattern, like a monthly newsletter (purple)
  2.  Natural Link Growth - A steady increase consistent with rising popularity (green)
  3.  Natural Link Growth - A mostly steady rate of link acquisition (blue)
  4.  Unnatural Link Growth - A one-time spike (red)
  5.  Negative Link Growth - A steadily falling rate of link acquisition consistent with falling popularity (brown)

It is also probable that each additional inlink has less influence. You could say the 10th external inlink is worth more than the 100th and that the 100th is worth more than the 1,000th.

External Link Distribution

The distribution of links to a domain is important too.

  •  On most web sites, nearly all the external links point to the home page or to a small number of pages.
  •  On better web sites there will be lots of links to lots of different pages. This signals to search engines that the domain has lots of different respected content. This is one reason why blogging can be an effective content and link building strategy.

Search engines look for many signs of natural linking or for manipulation. If your web site is found linking to other web sites unnaturally or benefiting from manipulative links, the search engines may penalize or ban your domain. Some unnatural links include:

  •  Reciprocal links - When two pages or domains link to each other.
  •  Site wide links - When every page on a web site links to another domain or to one page on another domain.
  •  Hidden links - Links that are not visible to people because they have no anchor text, are displayed in the same color as the background or made invisible with CSS.
  •  Paid links - Links that are purchased. (This does not include legitimate advertising or directory listings)

Context can play an important role in identifying unnatural links. For example, many blogs that cover the same subject areas and break new stories will frequently cite and link to each other. This is known as echoing. Search engines do not consider this reciprocal linking. In fact, if enough web sites link to each other frequently and other web sites link to them, the search engines can recognize these sites as a ball or circle of trust and give them dramatically increased authority.

External NoFollow Links

Search engines use links to measure trust, but their engineers do realize that you may need to link to a web site that you do not trust or have not been able to review. So you do not pass trust from your web site to these untrusted web sites search engines ask you to add the NoFollow attribute.

<a href=”” rel=”nofollow” >Anchor Text</a>

Search engines also ask that you use the NoFollow attribute on links for which you receive compensation.

Internal NoFollow Links

The NoFollow attribute can be used on internal links too in order to control the flow of authority passed by links. If you web site home page has x authority, and if it links to n pages, then each link passes (x/n)*d where d is a dampening factor. Dampening is necessary so authority cannot be passed forever and ever without end.

In theory you may want to NoFollow links to pages with little content. One example might be a contact page that only has a form.

You may also want to NoFollow links in your page templates that link deeper than the next level of pages down or to too far across into other topics or categories. This is consistent with maintaining good silo or pyramid style web site architecture.

Using NoFollow links internally is a relatively young practice. Some terrific debate exists about its efficacy and how best to implement the tactic. For example, a recent question asked if you NoFollow a menu link with generic anchor text and have a DoFollow link in the footer with contextually rich anchor text, will a search engine spider follow and index the second link? SEO theory experts and investigators are exploring these questions right now.

Measuring External Links

External links cannot be easily measured. While many SEO and search marketing companies have attempted to quantify link profiles into a simple and meaningful scale, it can actually be more accurate for an experienced SEO expert to look at the quantity and sources of links and to use his or her judgment.

Measuring Link Numbers

The only way to count the number of links pointing to a web page or domain is to crawl or spider the entire Internet. Only the major search engines and large technology firms have the resources to attempt this. For this reason we rely on data given to us by the search engines.


Many SEOs consider Yahoo! Site Explorer to be the best source for external links. It will give you a count of all external links and a list of up to 1,000 links. You can also download a tab delimited file of up to 1,000 external links.

The problem with Yahoo is that, if a web site has more than 1,000 links, many of the links that Yahoo reports can be from the same domain, especially if a domain receives site wide links.

There is also a question about how accurate Yahoo! Site Explorer’s numbers are. It can be argued that these two queries should show the same number of links, but they do not.

Google Webmaster Tools is a great source of external link information, but only for your own web sites. While you can get a terrific list of your own links, you cannot get information about competitors. This makes it useless for comparative reports.

Google regular search interface does have a link: operator, but it has been disabled for many years. For example, displays only three links. Even though Google’s link counts still appear in many tools the data is worthless.

  • Other web sites, such as All the Web, provide external link data, though their search indexes are usually provided by other search engines. For example, All the Web uses Yahoo’s index. Other sources seem to excel at finding made for advertising web sites.

Measuring Link Quality

How good is a link? Search Engine Optimizers have asked this question since Google created the links economy by marrying PageRank with its search engine. Most SEOs will agree that they would rather spend the same time attracting a few high quality links than large numbers of low quality links.

Unfortunately for us there are few signals of quality we can measure directly. For this reason we must rely on third parties. Here is a short list of examples.

  •  Google PageRank - Measures the likelihood that somebody will find a specific web page by hopping from page to page and only clicking on links. That definition is certainly inaccurate, but I think it captures the spirit of PageRank, so it will suffice as an informal descriptor.
  •  Yahoo! Site Explorer Page Links - How many pages link to the page that links to yours? You can also measure things link links to the entire domain and how deep a page is within its web site.
  •  Alexa, Quantcast and Compete are three web sites that try to rank every web site from most visited to least visited. Supposedly, a link from a web site on the top of their lists will be more valuable than a link from web sites low on their lists. None of these services is wholly accurate and you should take their exact results with skepticism. That said, if all three trend a web site highly or as growing in popularity then it is probably true.
  •  Links or citations in sites like or Digg or Technorati can indicate popularity.
  •  Domain Age - Search engines tend to like age factors because they cannot be faked.
  •  Does a web site have links from government or university or other types of web sites that search engines generally trust highly?

The list could and does go on and on. Any signal, factor or variable you can capture and measure is fair game. But what happens when a web site has 100,000 links or some other huge quantity? How can someone possibly assess every link? The answer is that you cannot. Which is why experience, intuition and gut feelings can be so important. It is one of the reasons I call SEO a craft, part science, part art and a lot of honing.

It’s getting late and this post has gotten long, so I’ll stop here. If I have created more questions than I answered, if I have confused you, or if you want to know more, ask away and I’ll try to answer your questions.