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what are the advantages and disadvantages of using search engine?
advantages
There are three very compelling advantages of most search engines.

The indexes of search engines are usually vast, representing significant portions of the Internet, offering a wide variety and quantity of information resources. 

The growing sophistication of search engine software enables us to precisely describe the information that we seek. 

disadvantages

1. Regardless of the growing sophistication, many well thought-out search phrases produce list after list of irrelevant web pages. The typical search still requires sifting through dirt to find the gems.  

2. Using search engines does involve a learning curve. Many beginning Internet users, because of these disadvantages, become discouraged and frustrated. 



What is an Invisible web or "Deep Web"?

The Deep Web (also called Deepnet, the invisible Web, dark Web or the hidden Web) refers to World Wide Web content that is not part of the Surface Web, which is indexed by standard search engines.

Mike Bergman, credited with coining the phrase,^[1] has said that searching on the Internet today can be compared to dragging a net across the surface of the ocean; a great deal may be caught in the net, but there is a wealth of information that is deep and therefore missed. Most of the Web's information is buried far down on dynamically generated sites, and standard search engines do not find it. Traditional search engines cannot "see" or retrieve content in the deep Web – those pages do not exist until they are created dynamically as the result of a specific search. The deep Web is several orders of magnitude larger than the surface Web.^[2]

how do you find invisible web?
Most of the information on the Invisible Web is maintained by academic institutions, and has a higher quality than search engine results. There are "academic gateways" that can help you find this information. The SJSU Academic Gateway is a fabulous resource that enables you to get into not only San Jose public libraries, but the San Jose State University library as well. In addition, there are governmental (US) databases such as Ask Eric, which provide access to over 3000 educational resources (organized by category), and the US Securities and Exchange Commission, which has given a whole new meaning to the phrase "a little light reading." 

why are there web pages not available in search engine or subject directories?
because no one had put it ,because its has not have the complete info. about the questions of the world. maybe no one still can find the answer...
compare and contrast individual search engines and mete search engines?
- they are useful to us because we can find info. easily and fast , they help us in reports,assignments, and so on.
they help us in our daily life
-they search engine is common and popular while meta search engines are not , and also search engine has many web. and meta search engines has not .
When is it appropriate to use a search engine?when is it appropriate to use a search/subject directory?  
when you need something to learn or something like assignments maybe reports, that you want it fast,quick and easy, this is the time to use it, we can search for new info. or new inventions or get updated easily.