Ever since search engines appeared on the internet, inbound links have been extremely important to search engine rankings. In the past, and up until the end of the 2000s, search engine analysts focused much more on link quantity than link quality.

Today, however, with the multiplicity of search engine algorithms and the proliferation of anti-spam technologies, quality has taken precedence. But just what is a quality link and what are the criteria to analyze if a link is of good quality? These questions are among the most important to SEO analysts as well as any marketing manager who wants to create partnerships with other websites and businesses or determine the health of their own organization’s website.

To evaluate the relevance and quality of a link, the analysis rests on on-site and off-site factors.

On-site factors include everything relating to the operation of a website itself such as all the elements related to programming, content, hosting, etc. It’s important to verify the onsite elements of a referring website (the site with a text hyperlink pointed towards yours), in order to detect any potential problems (the nature of which will be discussed later in this article).

Off-site factors consist of elements which do not directly relate to a website but which nevertheless considerably influence its SEO performance, such as inbound links and social signals.

The following list is not exhaustive, but it describes some of the essential factors to consider.

On-Site Factors

1. Website’s purpose: Above all, it is essential to examine if the website in question is pertinent and useful, or if its sole purpose is SEO (SEO press releases, link farms, low-quality directories, etc.). These sorts of websites are very low quality and any link pointing from them towards your site will not bring any added value. On the contrary, you will risk a Google penalty.

2. Presence of spam: An important reflex to have is to check for spam on the site, or to check if it has been hacked. The technique is quite simple: Just type the command “site:” on Google and verify if common spam keywords are present on the site. Example: “site: example.com viagara.” If results appear, verify them by checking the source code toeasily see if the website has been a victim of spam or not.

3. Theme: The site’s theme is of vital importance when considering the quality of the link. As much as possible, you must try and earn links coming from sites with a similar theme to yours.

4. Content: The content of the referring website is, without a doubt, one of the most important onsite elements to consider. When you analyze the quality of a link pointing to your website it is important to take a look at some of the site’s pages to see if it is offering high quality content. There are several elements of site content you can quickly analyze for this purpose. As an example, we’ll consider a blog. If you doubt the quality of the content of a blog which has linked to you, open at least 10 articles at random and analyze them. You should consider the following elements:

• Do some articles include links to questionable sites (questionable themes, sites with foreign domain extensions like .co.uk or .ru)?
• Do some links have over-optimized anchor text? If so, it’s likely that the links are for SEO purposes only and that the article has no value for users.
• Does the blog offer a large amount of sponsored posts or sell links in bulk?
• Are the articles sufficiently long or not?
• Is the content copied from other sites?

5. Link placement: The placement of your link on the referring site is very important. Google considers the presence of links in certain parts of the page grounds for penalizing your site. Ideally, a link should be placed directly in the content of an article with good SEO value. On the contrary, a link in the footer or sidebar of the page, especially if it is surrounded by other low-quality links, could negatively impact your site’s SEO performance.

An example of negative external links in a site’s footer:

6. Site indexation: When you’re looking to analyze the quality of a site on which you have an inbound link, you should of course ensure that the site is being indexed by search engines. If this isn’t the case, it is very possible that there are problems with the site. To verify if the site is being indexed, enter the following command into the search engine: “site:sitename.com.” If you notice that there are few results and that several pages aren’t being indexed, there is definitely a problem. 

7. Verification of source code: This element is slightly more technical and requires a person experienced in HTML. In the source code you can verify several elements. For example, you can check if search engine robots are blocked, if the site has been spammed or hacked, or if there are any elements present which can impact the site’s SEO performance. It happens more often than you think! 

8. Blog comments: Many blogs allow their readers to leave comments. Nevertheless, some blogs automatically publish comments without moderation, which results in an enormous amount of undesirable comments and spam, as seen in the below screen-capture. Avoid having a link to your site within unmoderated comment section at all costs because this can seriously impact your SEO performance. 

Example of a blog with un-moderated comments (almost 1400 spam comments): 

Off-site Factors 

1. Moz metrics : MozRank is a tool that calculates a link’s popularity score. The higher a site’s MozRank is, the more likely that the links it has towards other sites will transfer a strong authority. Thus, by analyzing the MozRank of a site that has linked to you, you should be able to get an idea of its quality

2. Domain history: To quickly analyze a link’s quality, you should always verify the domain’s history. For this, you will need two tools:

• Whois : A Whois tool, such as whois.domaintools.com, allows you to obtain different information on a website’s history, such as the date of domain name creation, the name of the registry and the host, as well as other useful information of this nature. The most important thing to know is the age of the domain name. If the domain is relatively young, it will not have a search engine history. On the contrary, if the domain name is very old (older than eight years), it will potentially have a lot of authority and search engine history.
• Wayback Machine : The Wayback Machine tool contains the history of millions of websites since 1996. With this tool, you can consult former versions of websites, from the creation of the domain name until today. This tool is very useful in order to know if the domain name has been used for “domain scraping”, or if it’s 100% legitimate. Links from expired high-authority domains which have been re-purchased by a web marketing specialist for SEO purposes should be avoided.
  
  
  

3. Link type: It is important to know that on the web, several types of possible links exist and not all of them have the same amount of SEO value. The four most common types of links are standard HTML links, HTML Nofollow links, JavaScript links and redirections.

• Standard HTML Link: Standard HTML links (< a href>) are those which are used the vast majority of the time. For example <href="http://www.example.com">Example</a>. This link has the most SEO impact. Google easily understands this type of link and takes it into consideration in its search algorithm.
• Nofollow HTML Link: HTML Nofollow links are identical to standard HTML links, except they contain the Nofollow attribute. Example: <a rel="nofollow" href="http://www.example.com/">example</a>. The Nofollow attribute tells search engines not to follow the link and therefore, to not take it into account in its ranking algorithm. Thus, these links do not carry any search engine authority. Nevertheless, Nofollow links are not useless. On the contrary, having a certain percentage of Nofollow links makes your link profile look more natural and therefore more relevant to search engines. This leaves you less vulnerable to penalization. 
• JavaScript Link: Recently, Google has been able to read JavaScript links, but this is not the case for all search engines. Also, Google’s ability to parse these links is not yet optimized. When it comes to inbound links, it is recommended to always use HTML links. JavaScript links should not be used except to link internally within a website.
• Redirection Link: In certain cases, the link referring to your site might pass through a 302 server-side redirection. 302 redirections (temporary redirections), are not taken into account by search engine algorithms. Thus, these links bring no value to your site. We recommend using the excellent Redirect Path plugin in order to determine the HTTP status code of your links.

4. Page Rank: Google Page Rank, which was formerly an important criterion to consider when analyzing a site, doesn’t hold as much importance today. The last PageRank update was in December 2013, more than 18 months ago. Since that time, numerous sites have acquired many quality links that aren’t included in Page Rank while many sites have lost links and link authority since then. It is important to consider, also, that all websites launched after December 2013 have a Page Rank of 0, even if they have excellent authority. Thus, today, a link from a PR 1 site can have much more impact than a link from a PR 4 site. Note that it is not detrimental to have an incoming link from a low-authority site if the site has low authority simply because it doesn’t have many incoming links. It is natural that such sites would link to various pages. On the other hand, it is detrimental to have links from sites that have low authority because they have been penalized. This is why it’s necessary to always verify incoming links from sites with Page Rank 0.  

5. Referring countries: Often neglected, the country of a referring website is one of the most important criteria to take into consideration when it comes to analyzing link quality. For example, why would an auto insurance agency in Montreal or a car dealership in Gatineau have many links from domains in France, Spain, and the UK, and few links from Canada? This would be totally unnatural. Consequently, this is one of the criteria used by Google in order to penalize sites that acquire links in a non-natural way. Certain countries nevertheless have exceptions. For example, it is totally legitimate for Canadian sites to have a lot of links coming from the United States, mostly because the majority of websites in Canada are hosted with companies in the United States (for example, GoDaddy or HostPapa). Tools such as whois.domaintools.com allow you to figure out the location where a website is hosted, as well as different information about its server. It wouldn’t be logical, however, for a Canadian site to receive many links from Germany or Russia.

6. Inbound link analysis: With the goal of determining the quality of an inbound link, it is absolutely necessary to analyze the link profile of referring websites. It is often at this step that potential problems can be detected. We strongly recommend using the tool Ahrefs. Here are some basic metrics to analyze:

• Link acquisition curve: If the curve is abnormal, there is a good chance that something strange is going on (unless the site just has really good buzz). A site that suddenly receives a large quantity of links is sometimes the sign of a serious problem, such as a spam attack or the mass acquisition of negative links.

Example of a normal acquisition curve:

Example of an abnormal acquisition curve:

• Number of links: The number of inbound links towards a website is a very subjective metric, because not every link has the same value. For example, a website with 100 high-quality links might carry much more authority than a site with 500 medium-quality links. Nevertheless, this metric could bring to light certain problems. For example, if a small Quebecois website that’s not very well-known has thousands of referring links, this would appear suspicious.
• Presence of .ru, .pl and .uk: Domain extensions ending in .ru, .pl and .uk are some of the most frequent sources of spam in the world. Often, when a site has a lot of links from those extensions, it’s an indication that spam is present. If this is the case, your link on the site won’t carry much value. 
• Presence of .edu and .gov: Links from .edu and .gov extensions are very difficult to acquire in a natural manner. Because Google grants these links a high authority, they are highly sought-out by spammers (comment spam, Wikis, hacking, etc.). If a site possesses many links of this type, it is important to verify the links to make sure they are genuine. If they are spammy there is a strong chance that all the site’s backlinks are toxic (even if the site’s PR and MozRank are high!). On the other hand, if the .edu and .gov links are legitimate, the site that is linking to you probably carries a lot of authority.
• Anchor text: If the anchor text on the site (text which links are situated on) seem bizarre or totally off-topic to you, the site might have been the victim of hacking or spam. A deeper analysis is needed in this case. In a usual case, a minimum of 40% of a site’s anchor text should be branded (meaning they consist of brand names, site names or URLs), and 40% are natural anchor texts such as “Click here, more information, see this website, etc.”

Example anchor text from a site that’s been the victim of spam: