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How Search Engines Work

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How To Use Web Search Engines
Tips on using internet search sites  like Google, alltheweb, and Yahoo.

Page 4 -- How Search Engines Work

What follows is a basic explanation of how search engines work.  For more detailed and technical information about current methods used by search engines like Google, check out our discussion of Search Engine Ranking Algorithms

Search engines use automated software programs knows as spiders or bots to survey the Web and build their databases. Web documents are retrieved by these programs and analyzed.  Data collected from each web page are then added to the search engine index.  When you enter a query at a search engine site, your input is checked against the search engine's index of all the web pages it has analyzed.  The best urls are then returned to you as hits, ranked in order with the best results at the top.

Keyword Searching

This is the most common form of text search on the Web.  Most search engines do their text query and retrieval using keywords. 

What is a keyword, exactly?  It can simply be any word on a webpage.  For example, I used the word "simply" in the previous sentence, making it one of the keywords for this particular webpage in some search engine's index.   However, since the word "simply" has nothing to do with the subject of this webpage (i.e., how search engines work), it is not a very useful keyword.   Useful keywords and key phrases for this page would be "search," "search engines," "search engine methods," "how search engines work," "ranking" "relevancy," "search engine tutorials," etc.  Those keywords would actually tell a user something about the subject and content of this page.

Unless the author of the Web document specifies the keywords for her document (this is possible by using meta tags), it's up to the search engine to determine them.  Essentially, this means that search engines pull out and index words that appear to be significant.  Since since engines are software programs, not rational human beings, they work according to rules established by their creators for what words are usually important in a broad range of documents.  The title of a page, for example, usually gives useful information about the subject of the page (if it doesn't, it should!).  Words that are mentioned towards the beginning of a document (think of the "topic sentence" in a high school essay, where you lay out the subject you intend to discuss) are given more weight by most search engines.   The same goes for words that are repeated several times throughout the document.

Some search engines index every word on every page. Others index only part of the document.  

Full-text indexing systems generally pick up every word in the text except commonly occurring stop words such as "a," "an," "the," "is," "and," "or," and "www."  Some of the search engines discriminate upper case from lower case; others store all words without reference to capitalization.

The Problem With Keyword Searching

Keyword searches have a tough time distinguishing between words that are spelled the same way, but mean something different (i.e. hard cider, a hard stone, a hard exam, and the hard drive on your computer). This often results in hits that are completely irrelevant to your query. Some search engines also have trouble with so-called stemming -- i.e., if you enter the word "big," should they return a hit on the word, "bigger?" What about singular and plural words? What about verb tenses that differ from the word you entered by only an "s," or an "ed"?

Search engines also cannot return hits on keywords that mean the same, but are not actually entered in your query. A query on heart disease would not return a document that used the word "cardiac" instead of "heart."

Refining Your Search

Most sites offer two different types of searches--"basic" and "refined" or "advanced."  In a "basic" search, you just enter a keyword without sifting through any pulldown menus of additional options.  Depending on the engine, though, "basic" searches can be quite complex.

Advanced search refining options differ from one search engine to another, but some of the possibilities include the ability to search on more than one word, to give more weight to one search term than you give to another, and to exclude words that might be likely to muddy the results.  You might also be able to search on proper names, on phrases, and on words that are found within a certain proximity to other search terms.  

Some search engines also allow you to specify what form you'd like your results to appear in, and whether you wish to restrict your search to certain fields on the internet (i.e., usenet or the Web) or to specific parts of Web documents (i.e., the title or URL).

Many, but not all search engines allow you to use so-called Boolean operators to refine your search. These are the logical terms AND, OR, NOT, and the so-called proximal locators, NEAR and FOLLOWED BY.

Boolean AND means that all the terms you specify must appear in the documents, i.e., "heart" AND "attack."  You might use this if you wanted to exclude common hits that would be irrelevant to your query. 

Boolean OR means that at least one of the terms you specify must appear in the documents, i.e., bronchitis, acute OR chronic.  You might use this if you didn't want to rule out too much.

Boolean NOT means that at least one of the terms you specify must not appear in the documents. You might use this if you anticipated results that would be totally off-base, i.e., nirvana AND Buddhism, NOT Cobain.

Not quite Boolean + and - Some search engines use the characters + and - instead of Boolean operators to include and exclude terms.

NEAR means that the terms you enter should be within a certain number of words of each other.  FOLLOWED BY means that one term must directly follow the other. ADJ, for adjacent, serves the same function. A search engine that will allow you to search on phrases uses, essentially, the same method (i.e., determining adjacency of keywords).

Phrases: The ability to query on phrases is very important in a search engine. Those that allow it usually require that you enclose the phrase in quotation marks, i.e., "space the final frontier."

Capitalization:  This is essential for searching on proper names of people, companies or products. Unfortunately, many words in English are used both as proper and common nouns--Bill, bill, Gates, gates, Oracle, oracle, Lotus, lotus, Digital, digital--the list is endless.

All the search engines have different methods of refining queries.  The best way to learn them is to read the help files on the search engine sites and practice!

Relevancy Rankings

Most of the search engines return results with confidence or relevancy rankings.  In other words, they list the hits according to how closely they think the results match the query.  However, these lists often leave users shaking their heads on confusion, since, to the user, the results may seem completely irrelevant.

Why does this happen?  Basically it's because search engine technology has not yet reached the point where humans and computers understand each other well enough to communicate clearly.

Most search engines use search term frequency as a primary way of determining whether a document is relevant.  If you're researching diabetes and the word "diabetes" appears multiple times in a Web document, it's reasonable to assume that the document will contain useful information.  Therefore, a document that repeats the word "diabetes" over and over is likely to turn up near the top of your list.

If your keyword is a common one, or if it has multiple other meanings, you could end up with a lot of irrelevant hits.  And if your keyword is a subject about which you desire information, you don't need to see it repeated over and over--it's the information about that word that you're interested in, not the word itself.

Some search engines consider both the frequency and the positioning of keywords to determine relevancy, reasoning that if the keywords appear early in the document, or in the headers, this increases the likelihood that the document is on target.  For example, one method is to rank hits according to how many times your keywords appear and in which fields they appear (i.e., in headers, titles or plain text).  Another method is to determine which documents are most frequently linked to other documents on the Web.  The reasoning here is that if other folks consider certain pages important, you should, too.

If you use the advanced query form on AltaVista, you can assign relevance weights to your query terms before conducting a search.  Although this takes some practice, it essentially allows you to have a stronger say in what results you will get back.

As far as the user is concerned, relevancy ranking is critical, and becomes more so as the sheer volume of information on the Web grows.  Most of us don't have the time to sift through scores of hits to determine which hyperlinks we should actually explore. The more clearly relevant the results are, the more we're likely to value the search engine.

Information On Meta Tags

Some search engines are now indexing Web documents by the meta tags in the documents' HTML (at the beginning of the document in the so-called "head" tag). What this means is that the Web page author can have some influence over which keywords are used to index the document, and even in the description of the document that appears when it comes up as a search engine hit.

This is obviously very important if you are trying to draw people to your website based on how your site ranks in search engines hit lists.

There is no perfect way to ensure that you'll receive a high ranking.  Even if you do get a great ranking, there's no assurance that you'll keep it for long.  For example, at one period a page from the Spider's Apprentice was the number- one-ranked result on Altavista for the phrase "how search engines work."   A few months later, however, it had dropped lower in the listings. 

There is a lot of conflicting information out there on meta-tagging.  If you're confused it may be because different search engines look at meta tags in different ways.  Some rely heavily on meta tags, others don't use them at all.  The general opinion seems to be that meta tags are less useful than they were a few years ago, largely because of the high rate of spamdexing (web authors using false and misleading keywords in the meta tags).

Note:  Google, currently the most popular search engine, does not index the keyword metatags.  Be aware of this is you are optimizing your webpages for the Google engine.

It seems to be generally agreed that the "title" and the "description" meta tags are important to write effectively, since several major search engines use them in their indices.   Use relevant keywords in your title, and vary the titles on the different pages that make up your website, in order to target as many keywords as possible.  As for the "description" meta tag, some search engines will use it as their short summary of your url, so make sure your description is one that will entice surfers to your site. 

Note:  The "description" meta tag is generally held to be the most valuable, and the most likely to be indexed, so pay special attention to this one.

In the keyword tag, list a few synonyms for keywords, or foreign translations of keywords (if you anticipate traffic from foreign surfers).  Make sure the keywords refer to, or are directly related to, the subject or material on the page.  Do NOT use false or misleading keywords in an attempt to gain a higher ranking for your pages.

The "keyword" meta tag has been abused by some webmasters.  For example, a recent ploy has been to put such words "sex" or "mp3" into keyword meta tags, in hopes of luring searchers to one's website by using popular keywords.

The search engines are aware of such deceptive tactics, and have devised various methods to circumvent them, so be careful.  Use keywords that are appropriate to your subject, and make sure they appear in the top paragraphs of actual text on your webpage.  Many search engine algorithms score the words that appear towards the top of your document more highly than the words that appear towards the bottom.  Words that appear in HTML header tags (H1, H2, H3, etc) are also given more weight by some search engines.  It sometimes helps to give your page a file name that makes use of one of your prime keywords, and to include keywords in the "alt" image tags.

One thing you should not do is use some other company's trademarks in your meta tags.  Some website owners have been sued for  trademark violations because they've used other company names in the meta tags.  I have, in fact, testified as an expert witness in such cases.  You do not want the expense of being sued!

Remember that all the major search engines have slightly different policies.  If you're designing a website and meta-tagging your documents, we recommend that you take the time to check out what the major search engines say in their help files about how they each use meta tags.  You might want to optimize your meta tags for the search engines you believe are sending the most traffic to your site. 

Concept-based searching (The following information is out-dated, but might have historical interest for researchers)

Excite used to be the best-known general-purpose search engine site on the Web that relies on concept-based searching.  It is now effectively extinct.

Unlike keyword search systems, concept-based search systems try to determine what you mean, not just what you say.  In the best circumstances, a concept-based search returns hits on documents that are "about" the subject/theme you're exploring, even if the words in the document don't precisely match the words you enter into the query.  

How did this method work?  There are various methods of building clustering systems, some of which are highly complex, relying on sophisticated linguistic and artificial intelligence theory that we won't even attempt to go into here.  Excite used to a numerical approach.  Excite's software determines meaning by calculating the frequency with which certain important words appear.  When several words or phrases that are tagged to signal a particular concept appear close to each other in a text, the search engine concludes, by statistical analysis, that the piece is "about" a certain subject.

For example, the word heart, when used in the medical/health context, would be likely to appear with such words as coronary, artery, lung, stroke, cholesterol, pump, blood, attack, and arteriosclerosis.  If the word heart appears in a document with others words such as flowers, candy, love, passion, and valentine, a very different context is established, and a concept-oriented search engine returns hits on the subject of romance.

This ends the outdated "concept-based" information section.

What does it all mean?

You now know more than you probably ever wanted to know about indexing, query refining and relevancy ranking.  How do we put it all together to make Web searching easier and more efficient than it currently is?

Let's try some practical applications.  It's time for:

The Web-Search Wizard

Find People on the Web (friends, celebrities, classmates, public figures)


Spidap, Top Page

Contact Us



The Spider's Apprentice was conceived and written by Linda Barlow, who maintains this site for Monash Information Services. Copyright, 1996-2004. All rights reserved.
Updated: 05/11/04