Search for anything using your favorite crawler-based search engine. Nearly instantly, the search
engine will sort through the millions of pages it knows about and present you with ones
that match your topic. The matches will even be ranked, so that the most relevant ones
come first.
Of course, the search engines don't always get it right. Non-relevant pages make it
through, and sometimes it may take a little more digging to find what you are looking for.
But, by and large, search engines do an amazing job.
As WebCrawler founder Brian Pinkerton puts it, "Imagine walking up to a librarian
and saying, ‘travel.’ They’re going to look at you with a blank face."
OK -- a librarian's not really going to stare at you with a vacant
expression. Instead, they're going to ask you questions to better
understand what you are looking for.
Unfortunately, search engines don't have the ability to ask a few questions to
focus your search, as a librarian can. They also can't rely on judgment and past experience to rank web pages,
in the way humans can.
So, how do crawler-based search engines go about determining relevancy,
when confronted with hundreds of millions of web pages to sort through? They follow a set of rules,
known as an algorithm. Exactly how a particular search engine's algorithm
works is a closely-kept trade secret. However, all major search engines
follow the general rules below.
Location, Location, Location...and Frequency
One of the the main rules in a ranking algorithm involves the location and frequency of keywords on a web page. Call
it the location/frequency method, for short.
Remember the librarian mentioned above? They need to find books to match your request
of "travel," so it makes sense that they first look at books with travel in the
title. Search engines operate the same way. Pages with the search terms appearing in the
HTML title tag are often assumed to be more relevant than others to the topic.
Search engines will also check to see if the search keywords appear near the top of a web
page, such as in the headline or in the first few paragraphs of text. They assume that any
page relevant to the topic will mention those words right from the beginning.
Frequency is the other major factor in how search engines determine relevancy. A search
engine will analyze how often keywords appear in relation to other words in a web page.
Those with a higher frequency are often deemed more relevant than other web pages.
Spice In The Recipe
Now it's time to qualify the location/frequency method described above. All the major
search engines follow it to some degree, in the same way cooks may follow a
standard chili recipe.
But cooks like to add their own secret ingredients. In the same way, search engines add
spice to the location/frequency method. Nobody does it exactly the same, which is
one reason why the
same search on different search engines produces different results.
To begin with, some search engines index more web pages than others. Some search
engines also index web pages more often than others. The result is that no search engine
has the exact same collection of web pages to search through. That naturally
produces differences, when comparing their results.
Meta tags are what many web designers mistakenly assume are the "secret" to
propelling their web pages to the top of the rankings. However, not all
search engines read meta tags. In addition, those that do read meta tags
may chose to weight them differently. Overall, meta tags can be part of the
ranking recipe, but they are not necessarily the secret
ingredient.
Search engines may also penalize pages or exclude them from the index, if they detect
search engine "spamming." An example is when a word is repeated hundreds of times on a page, to increase the frequency and propel the page higher in the listings. Search
engines watch for common spamming methods in a variety of ways, including following
up on complaints from their users.
Off The Page Factors
Crawler-based search engines have plenty of experience now with
webmasters who constantly rewrite their web pages in an attempt to gain
better rankings. Some sophisticated webmasters may even go to great
lengths to "reverse engineer" the location/frequency systems
used by a particular search engine. Because of this, all major search
engines now also make use of "off the page" ranking criteria.
Off the page factors are those that a webmasters cannot easily
influence. Chief among these is link analysis. By analyzing how pages link
to each other, a search engine can both determine what a page is about and
whether that page is deemed to be "important" and thus deserving
of a ranking boost. In addition, sophisticated techniques are used to
screen out attempts by webmasters to build "artificial" links
designed to boost their rankings.
Another off the page factor is clickthrough measurement. In short, this
means that a search engine may watch what results someone selects for a
particular search, then eventually drop high-ranking pages that aren't
attracting clicks, while promoting lower-ranking pages that do pull in
visitors. As with link analysis, systems are used to compensate for
artificial links generated by eager webmasters.
Learning More
The Search Engine Features Chart has a section that
summarizes key areas of how crawler-based search engines rank web pages. The Search
Engine Design Tips page also summarizes key tips that will help you improve the relevancy of your
pages with crawler-based search engines.
Search Engine Watch members
have access to the How
Search Engines Work section. This section provides detailed
information about how each major search engine gathers its listings and an
additional tips on enhancing your position in their results. Learn more
about becoming a Search Engine Watch member and the many benefits members
receive by visiting the Membership
page.
Reprinted from SearchEngineWatch.
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