Who or what, makes one page (that you might get in your search results) more useful than another one, so that it is put at the top of your search results?
The following answers both questions and was taken from http://www.monash.com/spidap4.html
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
- Keyword Searching
- Refining Your Search
- Relevancy Ranking
- Meta Tags
- Concept-based Searching (This information is dated, but might have historical interest for researchers)
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!
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.
What are some of your favourite search engines? Why do you like one more than the others?
I prefer to use google the most, it is bookmarked. I always use it when i want to find images and would go there first to use it to find general or international topics. google is the most common and is proven to be reliable. I also like www.ask.com or www.askjeeves.com, cause you can actually ask and type in full questions.
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