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 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.
