7 Web search and search engine
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7.1 Web search
7.1.1 Web background and history
Web is an access mode in Internet, but not the only one
Telnet
FTP
Usenet
…
About Web
The invention of hypertext, envisioned by Vannevar Bush in the 1940’s and first realized in working systems in the 1970’s, significantly precedes the formation of World Wide Web in the 1990’s
World Wide Web simply refers to as Web
Flexible and simple features has contributed enormously to the growth of the Web
Characteristics of Web
Large scale
No central control of authorship
Rapid develop
Extensible architecture
Robust HTML
Lack of coordination in its creation
Heterogeneity at a daunting scale
Diversity of backgrounds and motives of its participants
Simple open client-server architecture
Client-server architecture
Server communicates with the client via a protocol (the http or hypertext transfer protocol)
Simple, asynchronously carrying a variety of payloads encoded in HTML (HyperText Markup Language)
Client (generally a browser) can receive the server’s response
Client sends an http request to a web server through URL (Universal Resource Locator)
URL includes protocol, domain, path, Web page, …(query string)
Ignore what it does not understand
How big is the Web?
By the end of 1995, Altavista reported that it had crawled and indexed approximately 30 million static web pages
By the begin of 2008, Google declared it can index nearly 8 billion web pages, and Yahoo! declared 19 billion web pages
Web search engines have historically engaged in a battle of bragging rights over which one indexes more web pages
In fact, the actual totality is difficult to get
How to discover Web resources
Full-text index search engines
Keyword search interface
Inverted indexes
Ranking mechanisms
Taxonomies populated with web pages in categories
Browse through a hierarchical tree of category labels
The popularity declined over time
Yahoo!
Created by Jerry Yang and David Filo of Stanford University in 1994 April
Jerry & David＇s Guide to the World Wide Web
Begin with Web directory surpassing 1000 distinct nodes
The drawbacks of Web directory
Require manual editorial process
User’s idea should match that of the editors performing the classification
The development of search engine
Stage I:
Focusing on the challenge of scale
Documents, Web servers, …
Altavista, …
Stage II:
Improving the quality and relevance of web search results
Need to gauge the authoritativeness of a document
Google, …
Stage III:
Multimedia search, personalized search, …
7.1.2 Web graph
The definition of Web graph
Web consists of static HTML pages together with the hyperlinks between them as a directed graph in which each web page is a node and each hyperlink a directed edge
We refer to the set of all such nodes and directed edges as the web graph
Hyperlink
The characteristics of Web graph
Hyperlink is similar to document’ reference but not equal to it
Web graph is not strongly connected
In-links do not means out-links
The number of in-links to a page (also known as in-degree) has averaged from roughly 8 to 15
The distribution of links
Not follow the Poisson distribution
Every web page were not to pick the destinations of its links uniformly at random
Follow the power law
The total number of web pages with in-degree i is proportional to 1/iα
Web graph has a bowtie shape
Bowtie shape of Web graph
Referred to as IN, OUT, SCC, Tubes and Tendrils
SCC: strongly connected component
Tubes are small sets of pages outside SCC that lead directly from IN to OUT
Tendrils mean either leading nowhere from IN, or from nowhere to OUT
Any page in IN can reach any page in SCC by following hyperlinks
Likewise, any page in SCC can reach any page in OUT
Any page in SCC can reach any other page in SCC
Any page in SCC can not reach any page in IN
Any page in OUT can not reach any page in SCC
IN and OUT are roughly equal in size
SCC is somewhat largest
7.1.3 Economic issues
Spam and anti-spam
Web search advertising
7.1.3.1 Spam and anti-spam
Manipulation of web page content for the purpose of appearing high up in search results for selected keywords
Increase term frequencies
Hide these repeated terms
Different from the fact that a company uses large fonts to list its phone numbers in the yellow pages because the latter needs more cost
Even if company names beginning with a long string of A’s to be listed early in a yellow pages category
Paid inclusion
Auto Bidding (竞价排名)
Pay Per Click (点击付费)
Different search engines have different policies on whether to allow paid inclusion
More sophisticated spam
Cloaking
Doorway page
Link spam
Cloaking
The relationship between page publishers and web search engines is not completely collaborative
Doorway page
It contains text and metadata carefully chosen to rank highly on selected search keywords
When a browser requests the doorway page, it is redirected to a page containing content of amore commercial nature
Link spam
Manipulation of the links into a web page
SEO (Search Engine Optimizers)
Link farm
Search engines use link analysis to recognize link spam
Adversarial information retrieval has sprung up around this battle
7.1.3.2 Web search advertising
Sponsored search
Search advertising
Two types
Priced on a cost per mil (CPM) basis
Also known as impressions
Based on having its banner advertisement displayed 1000 times
Priced on the number of times it was clicked on by the user, known as the cost per click (CPC) model
The goal of the advertisement is not so much brand promotion as to induce a transaction
An example: Goto search engine
The pioneer in Web advertising based on CPC
Changed its name to Overture prior to eventual acquisition by Yahoo!
The market strategy of Goto
Every query term q it accepted bids from companies who wanted their web page shown on the query q
In response to the query q, Goto would return the pages of all advertisers who bid for q, ordered by their bids
When the user clicked on one of the returned results, the corresponding advertiser would make a payment to Goto
In the initial implementation, this payment equaled the advertiser’s bid for q
Combined strategy of current search engine
The primary response provides pure search results generally known as algorithmic search results
The sponsored search results displayed separately and distinctively to the right of the algorithmic results
Search engine marketing (SEM)
Understanding how search engines do this ranking and how to allocate marketing campaign budgets to different keywords and to different sponsored search engines has become a profession known as search engine marketing (SEM)
Click spam
Clicks on sponsored search results are not from bona fide search users
Two main types
From other devious advertising competitor
From devious search engine
7.1.4 The search user experience
The behavior of Web users
Web search users are not information scientists
Not know (or care) about the heterogeneity of web content, the syntax of query languages and the art of phrasing queries
The attracting strategy of search engine
Focus on relevance, specifically precision rather than recall in the first few results
Save users time in locating the information they sought
Believe that user experience is lightweight, meaning that both the search query page and the search results page are uncluttered and almost entirely textual, with very few graphical elements
Not bottlenecked by the time to load the search query or results page
The three types of user query needs
Informational queries
Navigational queries
Transactional queries
Informational queries
Seek general information on a broad topic
Not a single web page that contains all the information sought
User wants more web pages
Navigational queries
Seek the website or home page of a single entity that the user has in mind
In such cases, the user’s expectation is that the very first search result should be the home page about query term, not interested in documents containing the query term
The best measure of user satisfaction is precision at 1
Transactional queries
Prelude to the user performing a transaction on the Web – such as purchasing a product, downloading a file or making a reservation
The search engine should return results listing services that provide form interfaces for such transactions
How to distinguish?
Discerning which of these categories a query falls into can be challenging
It should be clear that some queries will fall in more than one of these categories, while others will fall outside them
[此贴子已经被作者于2010-12-14 09:22:17编辑过]
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