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Latent Semantic Indexing (LSI): What is it, and Why Should I Care?

Published by | 4 Comments

LSI (Latent Semantic Indexing) is a way that search engines determine whether your content is really on-topic and in-depth or just spam. The search engines determine this by looking at the words in an article and deciding how relevant they are to each other. For instance, for an article about computers, the search engines know that the following words are closely related to “computers” and will probably appear in any good article about computers: hard drive, cpu, RAM, monitor, motherboard, ghz, mhz, Intel, Nvidia, etc… These are known as LSI terms.

So, here’s what happens when the search engine finds your article:

1. It reads the article

2. It determines “keyword density” of each word or phrase in the article. This means that it looks at the entire number of words in the article and finds how many times particular words or phrases are repeated in the article. Words and phrases that are repeated more often have higher keyword density. This is how the search engine knows what your article is about. So for an article about “desktop PC cases,” that phrase might appear 4 times in your 700-word article. That would give it a density of about 1.7%. (12/700 = 1.7%).

We use 12 because “desktop PC cases” is comprised of 3 words, so 3 words X 4 appearances = 12. You can help the search engine figure out what your article is about by including the keyword in the title, first paragraph, and last paragraph of your article, as the search engines know to put extra emphasis on these areas of the article.

3. It picks out the words and phrases with the highest keyword density and uses those to determine what the article is about (in essence, the article is assigned a “relevancy score”). So for our “desktop PC cases” example, if it finds a high keyword density of “desktop PC cases” then it knows to expect high densities of other related terms (LSI terms), like: ATX, cooling, power supply, motherboard, gaming case, custom case, etc…

The search engines know what related terms to expect for any given keyword; they have gotten pretty smart. So if they expect to see certain related keywords in the article but they don’t find those keywords, they assign it a lower relevancy score. This directly impacts where that article will rank in the search engines when someone searches for your target keyword.

LSI is a key concept in SEO (search engine optimization). Search engine algorithms are always improving, and right now they are rewarding content that has a good balance of LSI terms with the main keyword of the article. So if your plan is to use the content that you write to build a Website, optimize that Website for SEO, and monetize the traffic that comes to the Website, it’s important that your articles have a good mix of LSI terms. So when I read articles, I read them from two different perspectives:

1. Human (Does it read well?)
2. SEO Specialist/search engine spider (Is there a good keyword density? Are there lots of LSI terms?)

So for future article marketing efforts, try to incorporate LSI terms. Just think about what terms are unique to the niche you’re writing for. Often, these terms will occur organically as you write. But for best results, you should be conscious of how the search engine spiders are going to read your content.

Take this blog post, for example. The main topic of this post is LSI (Latent Semantic Indexing), but I’ve sprinkled a bunch of LSI terms into this post:

  • SEO
  • content
  • relevant
  • keyword
  • keyword density
  • search engine
  • search engine optimization
  • Website
  • article marketing
  • rank
  • monetize

All these terms are related to LSI and the broader category of SEO, which the search engines will recognize when their spiders crawl this post. And the result? This blog post will be assigned a higher relevancy score for those categories. And that means it’ll rank higher in the search engine results pages (SERPs). Simple as that.

Now, don’t get me wrong; LSI isn’t the only determining factor of how your content ranks in the SERPs. In fact, it’s only one of hundreds, if not thousands of factors. That said, it’s gaining importance in the search engine algorithms so it’s worth thinking about when you are writing SEO-optimized content.

So, what is the #1 ranking factor?

Backlinks. To understand what backlinks are, how they play into the ranking algorithm, and how you can use this information to get your website ranked where you want it to be, see SEO Link Building: The Ultimate Step-by-Step Guide.

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Jayson DeMers

Jayson DeMers is the Founder & CEO of AudienceBloom. You can contact him on LinkedIn, Google+, or Twitter.

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  1. avatar


    Backlinks are not longer a ranking factor.
    Google has told us that they are no longer an “actionable metric”.

    Google has also told us that they do not use keyword density.
    What they do use is the text size and position to judge the importance of keywords.

    Headings and “h” tags provide the direction.

    LSI, or similar is used to determine relevance of text.

    • avatar

      Jayson DeMers

      Backlinks are still the most important ranking factor, and are absolutely an actionable metric. If Google said anything contradictory to this, I’d love to see where.

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