What is LSI and What Does it Mean for SEO?

Understanding LSI can help you write better web content(112521)Credit: Raja R.

LSI, or Latent Semantic Indexing, is a technology used by web search engines like Google to help determine the relevance and results page ranking of a single web page by comparing it with the millions of similar web pages all targeting the same keywords or keyphrases.  

The term for the method used in LSI is Latent Semantic Analysis (LSA), which is a mathematical analysis that draws out the underlying (“latent”) concepts in any number of documents by analyzing relationships between every word and phrasing contained within them.

In other words, if you do a search on Google for “healthy skin cream”, Google will use LSA to return web pages that not only include the keywords “healthy skin cream”, but also web pages that it has determined to be conceptually related to that search query – even if the keywords “healthy skin cream” don’t appear in those pages. If you’ve ever wondered why you sometimes get results from Google that don’t actually include any of your search terms, this is why.    

Important: LSI keywords are NOT the same thing as related queries. In the example above, the LSI keywords would be words like “soft”, “smooth”, “soothing”, and “natural”, because these are all words that are commonly found in web pages targeting the phrase “healthy skin cream”.

Related queries, on the other hand, simply refers to what other phrases are being frequently searched for based on the initial search query, such as “where to buy healthy skin cream” or “healthy skin cream for children”. 

So How Does LSI Work Exactly? How Can I Use LSI For My On-Page SEO?

LSI Keywords - SEOCredit: Svilen MilevUnless you are an incredibly smart and talented mathematician, learning the inner workings of Latent Semantic Indexing is a slightly pointless exercise. You can try, but the fact is, it’s highly complicated and certainly not worth the trouble if you are only interested in writing better search engine optimized content.

Here is an analogy that may help you better understand how LSI works from a practical on-page SEO perspective:

When writing an SEO-friendly article, you can think of Google as an English teacher that will be marking an essay you are writing and giving it a grade based on how well you discuss or answer the essay topic. If you write your essay without including any of the terminology specific to your topic, you simply won’t get a good grade because your teacher can’t tell if you’ve read and understood the concepts that are involved. For example, if you’re writing an essay on “The History of the Internet” you will have to know – and use – terms like “world wide web”, “TCP/IP”, “packet switching”, and so on.

On the flip-side, if you try to impress your teacher by writing about every possible aspect of your topic and including all the relevant terms you can get your hands on, you probably won’t get a very good grade either. Why? Because an essay is essentially a tightly-focused discussion on the topic at hand, and often requires little more than a few sound arguments and a conclusion.

Being comprehensive in dealing with your topic is great, but unless you are writing an encyclopedia entry,  it simply isn’t expected of you to include all available information in one single take – and this holds true for web content as well as for essays.  

A quick thought experiment to help you understand the importance of LSI in SEO

Imagine you have just written and published two articles on the web (we’ll call these Article A and Article B) of the same word count with exactly the same keywords in the page title and URL. In addition, your keywords are located in the same places in each article, so that you would have the same keywords once in the first paragraph, twice in the third paragraph, and once more in the final paragraph. And finally, both Article A and Article B are freshly published on the same website and have no backlinks as yet.  

Now, in terms of SEO (without Latent Semantic Indexing being applied yet), these articles should be ranked very near to each other in the search results for their keywords – assuming both articles were written in grammatically correct English and in the same general style.

And here's where LSI comes into play:

What if Article A, with the exact same keywords, keyword placement, and lack of backlinks as Article B, was written by an 80 year old whereas Article B was written by a 21 year old? The rest of the content of these articles – that is, all the words that aren’t the keywords – would be very different, right?

To demonstrate this point, let’s give both articles a specific title as well as an introductory sentence that highlights the differences in non-keyword content that you would expect to find between Article A and Article B.

For this example, the normal SEO keywords shown will be in bold, and the LSI keywords in italics.


Article A (80 year old writer)

Title: “My Honest Review of Market Samurai, The Best SEO Keyword Research Tool”

Intro: “As soon as I decided to write an honest review of Market Samurai, I turned on my data processor, made a connection to the series of tubes that lets me communicate with my family from far away, and double-clicked on the icon with the “I” and the “E”.


Article B (21 year old writer)

Title: “My Honest Review of Market Samurai, The Best SEO Keyword Research Tool”

Intro: “As soon as I decided to write an honest review of Market Samurai, I turned on my computer, connected to the internet, and loaded up my web browser.   


The example for Article A may be exaggerating a bit (most 80 year olds do in fact know what a computer is), but did you notice how little indication Article A’s first sentence gave on what the article is about?

Article B, on the other hand, said essentially the same thing but in different, LSI-friendly words like “computer”, “internet”, and “web browser” which are all semantically related to the topic, or theme, of online marketing. And this is why LSI keywords are important to consider when tweaking your articles with on-page SEO in mind.

So finally, and because LSI keywords are admittedly not the easiest concept to understand, let’s quickly recap what has been explained in this article.

Firstly, LSI keywords are not the same as related searches. You can use related search queries that Google serves up for ideas on what LSI keywords surround your initial results, but the related search queries by themselves are not automatically “LSI-worthy” keywords.

Secondly, don’t bother trying to understand the mathematics of LSI for SEO purposes. All you need is to understand the main concept behind it: the words you use in your articles should be tightly-focused, terminology-specific, and thematically-related to the topic you're writing about and you should be fine.

Now this is not to say you shouldn’t try to learn more about LSI and how to use LSI keywords effectively – just don’t get too caught up in the mechanics of it all and always aim to write for your readers as much as for the various search engine ranking algorithms.


Would you like to know more about using SEO in your writing? You should check out my previous article on this subject over here: Web Content Writing - A Freelance Beginner's Guide


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