Web Search and Analysis
The emergence of the Web has brought millions of users to search for information, which is stored in a very large number of active sites. To make this information accessible, search engines such as Google and Yahoo! have to crawl and index these sites and document collections in their index databases. Moreover, search engines have to regularly update their indexes given the dynamic nature of the Web as new Web sites are created and current ones are updated or deleted. Since there are many millions of pages available on the Web on different topics, search engines have to apply many sophisticated techniques such as link analysis to identify the importance of pages.
There are other types of search engines besides the ones that regularly crawl the Web and create automatic indexes: these are human-powered, vertical search engines or metasearch engines. These search engines are developed with the help of computer-assisted systems to aid the curators with the process of assigning indexes. They consist of manually created specialized Web directories that are hierarchically organized indexes to guide user navigation to different resources on the Web. Vertical search engines are customized topic-specific search engines that crawl and index a specific collection of documents on the Web and provide search results from that specific collection. Metasearch engines are built on top of search engines: they query different search engines simultaneously and aggregate and provide search results from these sources.
Another source of searchable Web documents is digital libraries. Digital libraries can be broadly defined as collections of electronic resources and services for the delivery of materials in a variety of formats. These collections may include a univer-sity’s library catalog, catalogs from a group of participating universities as in the State of Florida University System, or a compilation of multiple external resources on the World Wide Web such as Google Scholar or the IEEE/ACM index. These interfaces provide universal access to different types of content—such as books, articles, audio, and video—situated in different database systems and remote repos-itories. Similar to real libraries, these digital collections are maintained via a catalog and organized in categories for online reference. Digital libraries “include personal, distributed, and centralized collections such as online public access catalogs (OPACs) and bibliographic databases, distributed document databases, scholarly and professional discussion lists and electronic journals, other online databases, forums, and bulletin boards.”
1. Web Analysis and Its Relationship to Information Retrieval
In addition to browsing and searching the Web, another important activity closely related to information retrieval is to analyze or mine information on the Web for new information of interest. (We discuss mining of data from files and databases in Chapter 28.) Application of data analysis techniques for discovery and analysis of useful information from the Web is known as Web analysis. Over the past few years the World Wide Web has emerged as an important repository of information for many day-to-day applications for individual consumers, as well as a significant plat-form for e-commerce and for social networking. These properties make it an interesting target for data analysis applications. The Web mining and analysis field is an integration of a wide range of fields spanning information retrieval, text analysis, natural language processing, data mining, machine learning, and statistical analysis.
The goals of Web analysis are to improve and personalize search results relevance and to identify trends that may be of value to various businesses and organizations. We elaborate on these goals next.
Finding relevant information. People usually search for specific information on the Web by entering keywords in a search engine or browsing information portals and using services. Search services are constrained by search relevance problems since they have to map and approximate the information need of millions of users as an a priori task. Low precision (see Section 27.6) ensues due to results that are nonrelevant to the user. In the case of the Web, high recall (see section 27.6) is impossible to determine due to the inability to index all the pages on the Web. Also, measuring recall does not make sense since the user is concerned with only the top few documents. The most rele-vant feedback for the user is typically from only the top few results.
Personalization of the information. Different people have different content and presentation preferences. By collecting personal information and then generating user-specific dynamic Web pages, the pages are personalized for the user. The customization tools used in various Web-based applications and services, such as click-through monitoring, eyeball tracking, explicit or implicit user profile learning, and dynamic service composition using Web APIs, are used for service adaptation and personalization. A personalization engine typically has algorithms that make use of the user’s personalization information—collected by various tools—to generate user-specific search results.
Finding information of commercial value. This problem deals with finding interesting patterns in users’ interests, behaviors, and their use of products and services, which may be of commercial value. For example, businesses such as the automobile industry, clothing, shoes, and cosmetics may improve their services by identifying patterns such as usage trends and user preferences using various Web analysis techniques.
Based on the above goals, we can classify Web analysis into three categories: Web content analysis, which deals with extracting useful information/knowledge from Web page contents; Web structure analysis, which discovers knowledge from hyperlinks representing the structure of the Web; and Web usage analysis, which mines user access patterns from usage logs that record the activity of every user.
2. Searching the Web
The World Wide Web is a huge corpus of information, but locating resources that are both high quality and relevant to the needs of the user is very difficult. The set of Web pages taken as a whole has almost no unifying structure, with variability in authoring style and content, thereby making it more difficult to precisely locate needed information. Index-based search engines have been one of the prime tools by which users search for information on the Web. Web search engines crawl the Web and create an index to the Web for searching purposes. When a user specifies his need for information by supplying keywords, these Web search engines query their repository of indexes and produce links or URLs with abbreviated content as search results. There may be thousands of pages relevant to a particular query. A problem arises when only a few most relevant results are to be returned to the user. The discussion we had about querying and relevance-based ranking in IR systems in Sections 27.2 and 27.3 is applicable to Web search engines. These ranking algo-rithms explore the link structure of the Web.
Web pages, unlike standard text collections, contain connections to other Web pages or documents (via the use of hyperlinks), allowing users to browse from page to page. A hyperlink has two components: a destination page and an anchor text describing the link. For example, a person can link to the Yahoo! Website on his Web page with anchor text such as “My favorite Website.” Anchor texts can be thought of as being implicit endorsements. They provide very important latent human annotation. A person linking to other Web pages from his Web page is assumed to have some relation to those Web pages. Web search engines aim to distill results per their relevance and authority. There are many redundant hyperlinks, like the links to the homepage on every Web page of the Web site. Such hyperlinks must be eliminated from the search results by the search engines.
A hub is a Web page or a Website that links to a collection of prominent sites (authorities) on a common topic. A good authority is a page that is pointed to by many good hubs, while a good hub is a page that points to many good authorities. These ideas are used by the HITS ranking algorithm, which is described in Section 27.7.3. It is often found that authoritative pages are not very self-descriptive, and authorities on broad topics seldom link directly to one another. These properties of hyperlinks are being actively used to improve Web search engine result ranking and organize the results as hubs and authorities. We briefly discuss a couple of ranking algorithms below.
3. Analyzing the Link Structure of Web Pages
The goal of Web structure analysis is to generate structural summary about the Website and Web pages. It focuses on the inner structure of documents and deals with the link structure using hyperlinks at the interdocument level. The structure and content of Web pages are often combined for information retrieval by Web search engines. Given a collection of interconnected Web documents, interesting and informative facts describing their connectivity in the Web subset can be discovered. Web structure analysis is also used to reveal the structure of Web pages, which helps with navigation and makes it possible to compare/integrate Web page schemes. This aspect of Web structure analysis facilitates Web document classification and clustering on the basis of structure.
The PageRank Ranking Algorithm. As discussed earlier, ranking algorithms are used to order search results based on relevance and authority. Google uses the well-known PageRank algorithm, which is based on the “importance” of each page. Every Web page has a number of forward links (out-edges) and backlinks (in-edges). It is very difficult to determine all the backlinks of a Web page, while it is relatively straightforward to determine its forward links. According to the PageRank algorithm, highly linked pages are more important (have greater authority) than pages with fewer links. However, not all backlinks are important. A backlink to a page from a credible source is more important than a link from some arbitrary page. Thus a page has a high rank if the sum of the ranks of its backlinks is high. PageRank was an attempt to see how good an approximation to the “importance” of a page can be obtained from the link structure.
The computation of page ranking follows an iterative approach. PageRank of a Web page is calculated as a sum of the PageRanks of all its backlinks. PageRank treats the Web like a Markov model. An imaginary Web surfer visits an infinite string of pages by clicking randomly. The PageRank of a page is an estimate of how often the surfer winds up at a particular page. PageRank is a measure of query-independent impor-tance of a page/node. For example, let P(X) be the PageRank of any page X and C(X) be the number of outgoing links from page X, and let d be the damping factor in the range 0 < d < 1. Usually d is set to 0.85. Then PageRank for a page A can be calcu-lated as:
P(A) = (1 – d) + d (P(T1)/C(T1) + ... + P(Tn)/C(Tn))
Here T1, T2, ..., Tn are the pages that point to Page A (that is, are citations to page A). PageRank forms a probability distribution over Web pages, so the sum of all Web pages’ PageRanks is one.
The H ITS Ranking Algorithm. The HITS algorithm proposed by Jon Kleinberg is another type of ranking algorithm exploiting the link structure of the Web. The algorithm presumes that a good hub is a document that points to many hubs, and a good authority is a document that is pointed at by many other authorities. The algorithm contains two main steps: a sampling component and a weight-propagation component. The sampling component constructs a focused collection S of pages with the following properties:
S is relatively small.
S is rich in relevant pages.
S contains most (or a majority) of the strongest authorities.
The weight component recursively calculates the hub and authority values for each document as follows:
Initialize hub and authority values for all pages in S by setting them to 1.
While (hub and authority values do not converge):
For each page in S, calculate authority value = Sum of hub values of all pages pointing to the current page.
For each page in S, calculate hub value = Sum of authority values of all pages pointed at by the current page.
Normalize hub and authority values such that sum of all hub values in S equals 1 and the sum of all authority values in S equals 1.
4. Web Content Analysis
As mentioned earlier, Web content analysis refers to the process of discovering useful information from Web content/data/documents. The Web content data consists of unstructured data such as free text from electronically stored documents, semi-structured data typically found as HTML documents with embedded image data, and more structured data such as tabular data, and pages in HTML, XML, or other markup languages generated as output from databases. More generally, the term Web content refers to any real data in the Web page that is intended for the user accessing that page. This usually consists of but is not limited to text and graphics.
We will first discuss some preliminary Web content analysis tasks and then look at the traditional analysis tasks of Web page classification and clustering later.
Structured Data Extraction. Structured data on the Web is often very important as it represents essential information, such as a structured table showing the airline flight schedule between two cities. There are several approaches to structured data extraction. One includes writing a wrapper, or a program that looks for different structural characteristics of the information on the page and extracts the right con-tent. Another approach is to manually write an extraction program for each Website based on observed format patterns of the site, which is very labor intensive and time consuming. It does not scale to a large number of sites. A third approach is wrapper induction or wrapper learning, where the user first manually labels a set of train-ing set pages, and the learning system generates rules—based on the learning pages—that are applied to extract target items from other Web pages. A fourth approach is the automatic approach, which aims to find patterns/grammars from the Web pages and then uses wrapper generation to produce a wrapper to extract data automatically.
Web Information Integration. The Web is immense and has millions of documents, authored by many different persons and organizations. Because of this, Web pages that contain similar information may have different syntax and different words that describe the same concepts. This creates the need for integrating information from diverse Web pages. Two popular approaches for Web information integration are:
Web query interface integration, to enable querying multiple Web data-
bases that are not visible in external interfaces and are hidden in the “deep Web.” The deep Web consists of those pages that do not exist until they are created dynamically as the result of a specific database search, which produces some of the information in the page (see Chapter 14). Since traditional search engine crawlers cannot probe and collect information from such pages, the deep Web has heretofore been hidden from crawlers.
Schema matching, such as integrating directories and catalogs to come up with a global schema for applications. An example of such an application would be to combine a personal health record of an individual by matching and collecting data from various sources dynamically by cross-linking health records from multiple systems.
These approaches remain an area of active research and a detailed discussion of them is beyond the scope of this book. Consult the Selected Bibliography at the end of this chapter for further details.
Ontology-Based Information Integration. This task involves using ontologies to effectively combine information from multiple heterogeneous sources. Ontologies—formal models of representation with explicitly defined concepts and named relationships linking them—are used to address the issues of semantic heterogeneity in data sources. Different classes of approaches are used for information integration using ontologies.
Single ontology approaches use one global ontology that provides a shared vocabulary for the specification of the semantics. They work if all informa-tion sources to be integrated provide nearly the same view on a domain of knowledge. For example, UMLS (described in Section 27.4.3) can serve as a common ontology for biomedical applications.
In a multiple ontology approach, each information source is described by its own ontology. In principle, the “source ontology” can be a combination of several other ontologies but it cannot be assumed that the different “source ontologies” share the same vocabulary. Dealing with multiple, partially over-lapping, and potentially conflicting ontologies is a very difficult problem faced by many applications, including those in bioinformatics and other complex area of knowledge.
Hybrid ontology approaches are similar to multiple ontology approaches: the semantics of each source is described by its own ontology. But in order to make the source ontologies comparable to each other, they are built upon one global shared vocabulary. The shared vocabulary contains basic terms (the primitives) of a domain of knowledge. Because each term of source ontology is based on the primitives, the terms become more easily compara-ble than in multiple ontology approaches. The advantage of a hybrid approach is that new sources can be easily added without the need to modify the mappings or the shared vocabulary. In multiple and hybrid approaches, several research issues, such as ontology mapping, alignment, and merging, need to be addressed.
Building Concept Hierarchies. One common way of organizing search results is via a linear ranked list of documents. But for some users and applications, a better way to display results would be to create groupings of related documents in the search result. One way of organizing documents in a search result, and for organiz-ing information in general, is by creating a concept hierarchy. The documents in a search result are organized into groups in a hierarchical fashion. Other related techniques to organize docments are through classification and clustering (see Chapter 28). Clustering creates groups of documents, where the documents in each group share many common concepts.
Segmenting Web Pages and Detecting Noise. There are many superfluous parts in a Web document, such as advertisements and navigation panels. The information and text in these superfluous parts should be eliminated as noise before classifying the documents based on their content. Hence, before applying classification or clustering algorithms to a set of documents, the areas or blocks of the documents that contain noise should be removed.
5. Approaches to Web Content Analysis
The two main approaches to Web content analysis are
(1) agent based (IR view) and
(2) database based (DB view).
The agent-based approach involves the development of sophisticated artificial intelligence systems that can act autonomously or semi-autonomously on behalf of a particular user, to discover and process Web-based information. Generally, the agent-based Web analysis systems can be placed into the following three categories:
Intelligent Web agents are software agents that search for relevant information using characteristics of a particular application domain (and possibly a user profile) to organize and interpret the discovered information. For example, an intelligent agent that retrieves product information from a variety of vendor sites using only general information about the product domain.
Information Filtering/Categorization is another technique that utilizes Web agents for categorizing Web documents. These Web agents use methods from information retrieval, and semantic information based on the links among various documents to organize documents into a concept hierarchy.
Personalized Web agents are another type of Web agents that utilize the personal preferences of users to organize search results, or to discover information and documents that could be of value for a particular user. User preferences could be learned from previous user choices, or from other individuals who are considered to have similar preferences to the user.
The database-based approach aims to infer the structure of the Website or to trans-form a Web site to organize it as a database so that better information management and querying on the Web become possible. This approach of Web content analysis primarily tries to model the data on the Web and integrate it so that more sophisticated queries than keyword-based search can be performed. These could be achieved by finding the schema of Web documents, building a Web document ware-house, a Web knowledge base, or a virtual database. The database-based approach may use a model such as the Object Exchange Model (OEM) that represents semi-structured data by a labeled graph. The data in the OEM is viewed as a graph, with objects as the vertices and labels on the edges. Each object is identified by an object identifier and a value that is either atomic—such as integer, string, GIF image, or HTML document—or complex in the form of a set of object references.
The main focus of the database-based approach has been with the use of multilevel databases and Web query systems. A multilevel database at its lowest level is a data-base containing primitive semistructured information stored in various Web repositories, such as hypertext documents. At the higher levels, metadata or generalizations are extracted from lower levels and organized in structured collections such as relational or object-oriented databases. In a Web query system, information about the content and structure of Web documents is extracted and organized using database-like techniques. Query languages similar to SQL can then be used to search and query Web documents. They combine structural queries, based on the organization of hypertext documents, and content-based queries.
6. Web Usage Analysis
Web usage analysis is the application of data analysis techniques to discover usage patterns from Web data, in order to understand and better serve the needs of Web-based applications. This activity does not directly contribute to information retrieval; but it is important to improve or enhance the users’ search experience. Web usage data describes the pattern of usage of Web pages, such as IP addresses, page references, and the date and time of accesses for a user, user group, or an application. Web usage analysis typically consists of three main phases: preprocessing, pattern discovery, and pattern analysis.
Preprocessing. Preprocessing converts the information collected about usage statistics and patterns into a form that can be utilized by the pattern discovery methods. We use the term “page view” to refer to pages viewed or visited by a user. There are several different types of preprocessing techniques available:
Usage preprocessing analyzes the available collected data about usage pat-terns of users, applications, and groups of users. Because this data is often incomplete, the process is difficult. Data cleaning techniques are necessary to eliminate the impact of irrelevant items in the analysis result. Frequently, usage data is identified by an IP address, and consists of clicking streams that are collected at the server. Better data is available if a usage tracking process is installed at the client site.
Content preprocessing is the process of converting text, image, scripts and other content into a form that can be used by the usage analysis. Often, this consists of performing content analysis such as classification or clustering. The clustering or classification techniques can group usage information for similar types of Web pages, so that usage patterns can be discovered for specific classes of Web pages that describe particular topics. Page views can also be classified according to their intended use, such as for sales or for discovery or for other uses.
Structure preprocessing: The structure preprocessing can be done by parsing and reformatting the information about hyperlinks and structure between viewed pages. One difficulty is that the site structure may be dynamic and may have to be constructed for each server session.
The techniques that are used in pattern discovery are based on methods from the fields of statistics, machine learning, pattern recognition, data analysis, data mining, and other similar areas. These techniques are adapted so they take into consideration the specific knowledge and characteristics for Web Analysis. For example, in association rule discovery (See Section 28.2), the notion of a transaction for market-basket analysis considers the items to be unordered. But the order of accessing of Web pages is important, and so it should be considered in Web usage analysis. Hence, pattern discovery involves mining sequences of page views. In general, using Web usage data, the following types of data mining activities may be performed for pattern discovery.
Statistical analysis. Statistical techniques are the most common method to extract knowledge about visitors to a Website. By analyzing the session log, it is possible to apply statistical measures such as mean, median, and frequency count to parameters such as pages viewed, viewing time per page, length of navigation paths between pages, and other parameters that are relevant to Web usage analysis.
Association rules. In the context of Web usage analysis, association rules refer to sets of pages that are accessed together with a support value exceed-ing some specified threshold. (See Section 28.2 on association rules.) These pages may not be directly connected to one another via hyperlinks. For example, association rule discovery may reveal a correlation between users who visited a page containing electronic products to those who visit a page about sporting equipment.
Clustering. In the Web usage domain, there are two kinds of interesting clusters to be discovered: usage clusters and page clusters. Clustering of users tends to establish groups of users exhibiting similar browsing patterns.
Such knowledge is especially useful for inferring user demographics in order to perform market segmentation in E-commerce applications or provide personalized Web content to the users. Clustering of pages is based on the content of the pages, and pages with similar contents are grouped together. This type of clustering can be utilized in Internet search engines, and in tools that provide assistance to Web browsing.
Classification. In the Web domain, one goal is to develop a profile of users belonging to a particular class or category. This requires extraction and selection of features that best describe the properties of a given class or cate-gory of users. As an example, an interesting pattern that may be discovered would be: 60% of users who placed an online order in /Product/Books are in the 18-25 age group and live in rented apartments.
Sequential patterns. These kinds of patterns identify sequences of Web accesses, which may be used to predict the next set of Web pages to be accessed by a certain class of users. These patterns can be used by marketers to produce targeted advertisements on Web pages. Another type of sequen-tial pattern pertains to which items are typically purchased following the purchase of a particular item. For example, after purchasing a computer, a printer is often purchased
Dependency modeling. Dependency modeling aims to determine and model significant dependencies among the various variables in the Web domain. As an example, one may be interested to build a model representing the different stages a visitor undergoes while shopping in an online store based on the actions chosen (e.g., from a casual visitor to a serious potential buyer).
The final step is to filter out those rules or patterns that are considered to be not of interest from the discovered patterns. The particular analysis methodology based on the application. One common technique for pattern analysis is to use a query language such as SQL to detect various patterns and relationships. Another technique involves loading of usage data into a data ware-house with ETL tools and performing OLAP operations to view it along multiple dimensions (see Section 29.3). It is common to use visualization techniques, such as graphing patterns or assigning colors to different values, to highlight patterns or trends in the data.
7. Practical Applications of Web Analysis
Web Analytics. The goal of web analytics is to understand and optimize the performance of Web usage. This requires collecting, analyzing, and performance monitoring of Internet usage data. On-site Web analytics measures the performance of a Website in a commercial context. This data is typically compared against key performance indicators to measure effectiveness or performance of the Website as a whole, and can be used to improve a Website or improve the marketing strategies.
Web Spamming. It has become increasingly important for companies and individuals to have their Websites/Web pages appear in the top search results. To achieve this, it is essential to understand search engine ranking algorithms and to present the information in one’s page in such a way that the page is ranked high when the respective keywords are queried. There is a thin line separating legitimate page optimization for business purposes and spamming. Web Spamming is thus defined as a deliberate activity to promote one’s page by manipulating the results returned by the search engines. Web analysis may be used to detect such pages and discard them from search results.
Web Security. Web analysis can be used to find interesting usage patterns of Websites. If any flaw in a Website has been exploited, it can be inferred using Web analysis thereby allowing the design of more robust Websites. For example, the backdoor or information leak of Web servers can be detected by using Web analysis techniques on some abnormal Web application log data. Security analysis techniques such as intrusion detection and denial of service attacks are based on Web access pattern analysis.
Web Crawlers. Web crawlers are programs that visit Web pages and create copies of all the visited pages so they can be processed by a search engine for indexing the downloaded pages to provide fast searches. Another use of crawlers is to automatically check and maintain the Websites. For example, the HTML code and the links in a Website can be checked and validated by the crawler. Another unfortunate use of crawlers is to collect e-mail addresses from Web pages, so they can be used for spam e-mails later.
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