Abstract:
A classifier for semi-structured documents and associated method dynamically and accurately classify documents with an implicit or explicit schema by taking advantage of the term-frequency and term distribution information inherent in the document. The system uses a structured vector model that allows like terms to be grouped together and dissimilar terms to be segregated based on their frequency and distribution within the sub-vectors of the structure vector, thus achieving context sensitivity. The final decision for assigning the class of a document is based on a mathematical comparison of the similarity of the terms in the structured vector to those of the various class models. The classifier of the present invention is capable of both learning and testing. In the learning phase the classifier develops models for classes with information it develops from the composite information gleaned from numerous training documents. Specifically, it develops a structured vector model for each training document. Then, within a given class of documents it adds and then normalizes the occurrences of terms.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the field of automated information retrieval in the context of document characterization and classification. Particularly, the present invention relates to a system and associated method for classifying semi-structured data maintained in systems that are linked together over an associated network such as the Internet. More specifically, this invention pertains to a computer software product for dynamically categorizing and classifying documents by taking advantage of both textual information as well as latent information embedded in the structure or schema of the documents, in order to classify their contents with a high degree of precision. This invention incorporates a structured vector model, and relies on a document classifier that assumes a structured vector model. 
     BACKGROUND OF THE INVENTION 
     The World Wide Web (WWW) is comprised of an expansive network of interconnected computers upon which businesses, governments, groups, and individuals throughout the world maintain inter-linked computer files known as web pages. The phenomenal growth of the WWW has led to the proliferation of data in semi-structured formats such as HTML and XML. There is a pressing need to support efficient and effective information retrieval, search and filtering. An accurate classifier is an essential component of building a semi-structured database system. 
     Currently, users navigate Web pages by means of computer software programs/search tools that commonly fall into two broad categories: net directories and search engines. Net directories provide a hierarchical classification of documents based on a manual classification of Web page materials and data. Search engines use a keyword-based search methodology to return to the user a set of pages that contain a given keyword or words. Both search tools suffer from significant limitations. Net directories are precise but are very limited in scope and expensive to maintain, primarily because of the requirement for human effort to build and maintain them. Search engines are more capable of covering the expanse of the Web but suffer from low precision and in their current embodiments, are reaching their logical limits. Search engines may provide to the user a null return or, conversely, a multitude of responses, the majority of which are irrelevant. 
     A number of techniques have been applied to the problem. Among them: statistical decision theory, machine learning, and data mining. Probabilistic classifiers use the joint probabilities of words and categories to estimate the probability of a document falling in a given category. These are the so-called term-based classifiers. Neural networks have been applied to text categorization. Decision tree algorithms have been adapted for data mining purposes. 
     The problems associated with automated document classification are manifold. The nuances and ambiguity inherent in language contribute greatly to the lack of precision in searches and difficulty of achieving successful automated classification of documents. For example, it is quite easy for an English-speaking individual to differentiate between the meanings of the word “course” in the phrase “golf course” and the phrase “of course.” A pure, term-based classifier, incapable of interpreting contextual meaning, would wrongly lump the words into the same category and reach a flawed conclusion about a document that contained the two phrases. Another difficulty facing automatic classifiers is the fact that all terms are not equal from a class standpoint. 
     Certain terms are good discriminators because they occur significantly more in one class than another. Other terms must be considered noise because they occur in all classes almost indifferently. The effective classifier must be able to effectively differentiate good discriminators from noise. Yet another difficulty for classifiers is the evaluation of document structure and relative importance of sections within the document. As an example, for a classifier dealing with resumes, sections on education and job skills would need to be recognized as being more important than hobbies or personal background. 
     These and other language problems represent difficulties for automated classification of documents of any type, but the World Wide Web introduces its own set of problems as well. Among these problems are the following: 
     
       
         
               
               
             
           
               
                   
               
             
             
               
                 1. 
                 Web documents are extremely diverse in content, structure, style 
               
               
                   
                 and format, partly because of their diverse authorship. Many 
               
               
                   
                 of the techniques that have been developed are only effective on 
               
               
                   
                 documents with homogeneous corpora. 
               
               
                 2. 
                 A significant fraction of Web documents are hypertext documents, 
               
               
                   
                 often divided into pages that are connected by hyperlinks. 
               
               
                   
                 Documents used for most existing Information Retrieval (IR) 
               
               
                   
                 studies are self-contained and cannot deal with the links. 
               
               
                 3. 
                 Most popular web document formats such as HTML or XML are 
               
               
                   
                 semi-structured, implying either an explicit or implicit, 
               
               
                   
                 though not fixed, schema. Previous Information Retrieval (IR) 
               
               
                   
                 efforts have focused on flat (unstructured) documents. The 
               
               
                   
                 markups and formatting cues in the document can mislead the 
               
               
                   
                 classifiers; removing or ignoring them means that only part 
               
               
                   
                 of the original information is available for classification. 
               
               
                   
               
             
          
         
       
     
     The challenges, then, are to deal with the problems inherent in all documents but to also deal with the special problems associated with Web documents, in particular those with a semi-structured format. 
     As noted, semi-structured data are data that do not have a fixed schema. Semi-structured data, however, have a schema, either implicit or explicit, but do not have to conform to a fixed schema. By extension, semi-structure documents are text files that contain semi-structured data. Examples include documents in HTML and XML and, thus, represent a large fraction of the documents on the Web. 
     The exploitation of the features inherent in such documents is a key to attaining and obtaining better information retrieval is not new. For example, one classifier has been designed to specifically take advantage of the hyperlinks available in HTML. Reference is made to Soumen Chakrabarti, et al., “Enhanced Hypertext Categorization Using Hyperlinks,” Proc. of ACM SIGMOD Conference, pages 307-318, Seattle, Wash., 1998. 
     In this manner, the classifier can evaluate for both and non-local data information to better categorize a document. However, there are more features of semi-structured documents that can be used for classification along with new techniques for evaluating the information gleaned from the documents. 
     Currently, there exists no other classifier that takes full advantage of the information available in semi-structured documents to produce accurate classification of such documents residing on the World Wide Web. The need for such a classifier has heretofore remained unsatisfied. 
     SUMMARY OF THE INVENTION 
     The text classifier for semi-structured documents and associated method of the present invention satisfy this need. In accordance with one embodiment, the system can dynamically and accurately classify documents with an implicit or explicit schema by taking advantage of the term-frequency and term distribution information inherent in the document. The system further uses a structured vector model that allows like terms to be grouped together and dissimilar terms to be segregated based on their frequency and distribution within the sub-vectors of the structure vector, thus achieving context sensitivity. The final decision for assigning the class of a document is based on a mathematical comparison of the similarity of the terms in the structured vector to those of the various class models. 
     The classifier of the present invention is capable of both learning and testing. In the learning phase the classifier develops models for classes with information it develops from the composite information gleaned from numerous training documents. Specifically, it develops a structured vector model for each training document. Then, within a given class of documents it adds and then normalizes the occurrences of terms. 
     The classifier further employs a feature selection technique to differentiate between good discriminators and noise and to discard noise terms on the basis of the structure the terms appear. It additionally employs a feature selection technique that determines the relative importance of sections of textual information. Once models for classes have been developed, the classifier can be used on previously unseen documents to assign best matching classes by employing a robust statistical algorithm. 
     To fully appreciate the characteristics and capabilities of the classifier it is first important to understand the basic characteristics of an XML (or other semi-structured) document and, further, to understand the concept of the extended model required to exploit the information encoded in them. XML documents differ from typical text documents in the following respects: 
     
       
         
               
               
             
           
               
                   
               
             
             
               
                 1. 
                 Each element of an XML document is tagged. 
               
               
                 2. 
                 The tags are usually nested, making the XML documents 
               
               
                   
                 hierarchical in nature. 
               
               
                 3. 
                 Any element of an XML document can be referred to by 
               
               
                   
                 any other element, thus requiring the documents to be 
               
               
                   
                 modeled as directed graphs. Under some circumstances, 
               
               
                   
                 a special case of a directed graph, a tree, can be used, 
               
               
                   
                 particularly for purposes of exposition. 
               
               
                   
               
             
          
         
       
     
     Proper classification of XML documents, thus, requires a scheme that exploits the rich information encoded in their structure. It is necessary to extend the notion of a document to incorporate the hierarchical sectioning of text. In an extended model, a document is hierarchically structured and text is embedded in the structure. 
     The hierarchical structure can be understood in the context of the analogy to a book. A book consists of many chapters, which, in turn, consist of many sections formed of many sentences, which, in turn, consist of many words. A word belongs to a sentence that contains the word, thus to the section that contains the sentence, the chapter that contains the section and, ultimately, to the book at its highest level. Thus, in a structured document a term (or equivalently a leaf or text) belongs to its antecedents. In the parlance of graph theory, the leaf belongs to its parent, its grandparent and all higher ancestors, ultimately belonging to the document or root. 
     The structure of the model is based on the following observation: Terms from the same XML element have to be grouped together to be treated in the same way, and to be differentiated from terms in other XML elements. The primary reason is that terms in one substructure may have a distribution that is different from another substructure or different distribution of terms from the overall document. By taking into account the structural information the classifier can achieve a context sensitivity that flat (unstructured) document models cannot achieve. 
     The algorithm used by the classifier may be summarized by the following process: 
     
       
         
               
               
             
           
               
                   
               
             
             
               
                 1. 
                 For each term, augment the term by the tags of the elements 
               
               
                   
                 at various levels of the paths. 
               
               
                 2. 
                 Compute the F score of each augmented term. The F score 
               
               
                   
                 measures the term&#39;s ability to separate classis as a ration 
               
               
                   
                 of inter-class to intra-class variation. 
               
               
                 3. 
                 Order the terms by decreasing order of F score and pick the 
               
               
                   
                 first k terms from the sequence of terms from each structure 
               
               
                   
                 (i.e., the terms with same prefix after term augmentation). 
               
               
                   
                 K is a given threshold. 
               
               
                 4. 
                 Select document structure elements of which the average F 
               
               
                   
                 value of terms is greater than the given threshold q. 
               
               
                 5. 
                 For classification of semi-structured documents, the class 
               
               
                   
                 is chosen that has the most similar terms distribution to 
               
               
                   
                 the term distribution of the test document. The term 
               
               
                   
                 distribution is the distribution of augmented terms. 
               
               
                   
               
             
          
         
       
     
     The semi-structured document classifier of the present invention provides several features and advantages, among which are the following: 
     
       
         
               
               
             
           
               
                   
               
             
             
               
                 1. 
                 It generates and uses a structure known as a structured 
               
               
                   
                 vector model that contains sub-vectors corresponding to 
               
               
                   
                 the elements within the document. 
               
               
                 2. 
                 Unlike previous classifiers, a term is treated 
               
               
                   
                 differently if it belongs to a different sub-vector. 
               
               
                   
                 Terms belonging to a different sub-vector are treated 
               
               
                   
                 differently, receiving different weightings. Thus, 
               
               
                   
                 it achieves a context sensitivity that leads to 
               
               
                   
                 significant improvements over text-based classifiers. 
               
               
                 3. 
                 It takes into account the term frequency and distribution 
               
               
                   
                 within a document as a whole but also within the 
               
               
                   
                 sub-structures of the document. 
               
               
                 4. 
                 It is able to utilize information latent in document 
               
               
                   
                 structures. 
               
               
                 5. 
                 It considers the document as a tree with a hierarchy 
               
               
                   
                 of terms rather than a simple collection of terms. 
               
               
                 6. 
                 It uses a process known as tag augmentation to encode 
               
               
                   
                 the tagging and nesting of elements in a semi-structured 
               
               
                   
                 document. It explicitly groups like structure elements 
               
               
                   
                 of terms together and, further, segregates dissimilar terms. 
               
               
                 7. 
                 It bases its final decision on the topics of the document 
               
               
                   
                 on both the term frequency and term distribution 
               
               
                   
                 information and structure of the document as described above. 
               
               
                 8. 
                 It achieves the benefits enumerated for a structured model 
               
               
                   
                 and the speed of a flat vector model by taking advantage of 
               
               
                   
                 the equivalence of tag augmentation and the structured vector 
               
               
                   
                 model. 
               
               
                 9. 
                 It incorporates context-sensitive term selection in its 
               
               
                   
                 learning algorithm in the context of document structure. 
               
               
                   
                 Term selection is performed for each sub-structure of 
               
               
                   
                 document. That is, it differentiates between significant 
               
               
                   
                 terms and noise per structure basis instead of the entire 
               
               
                   
                 document. 
               
               
                 10. 
                 It relies on a well-defined, robust probability function 
               
               
                   
                 to make a final determination the class of a document. 
               
               
                   
                 The classifier is the first to incorporate both textual 
               
               
                   
                 and structural features into the statistical model or 
               
               
                   
                 class determination. 
               
               
                   
               
             
          
         
       
     
     The foregoing and other features and advantages of the present invention are realized by a classifier that takes advantages of the hierarchical nature of documents exemplified by those in XML (extensible Markup Language), or any other language whose structure is hierarchical in nature and includes tags with each element. The classifier presented herein uses the inherent structure of XML or other semi-structured documents to provide high quality semantic clues that may not be otherwise taken advantage of by term-based classification schemes. The classifier further relies on a robust statistical model and a structure-based context-sensitive feature for better classification 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The various features of the present invention and the manner of attaining them will be described in greater detail with reference to the following description, claims, and drawings, wherein reference numerals are reused, where appropriate, to indicate a correspondence between the referenced items, and wherein: 
     FIG. 1 is a schematic illustration of an exemplary operating environment in which a classifier for semi-structured documents of the present invention can be used; 
     FIG. 2 is a block diagram that illustrates a high level architecture of the system of FIG. 1 shown used in the context of an Internet search; 
     FIG. 3 is an illustration of a non-structured vector used by a conventional classifier; 
     FIG. 4 is a structured vector model of the present invention used in the classifier of FIG. 2; 
     FIG. 5 depicts the classifier using a classification algorithm based on the structured vector model of FIG. 4; 
     FIG. 6 is block diagram illustrating the training and testing phases for use by the classification algorithm of FIG. 5; 
     FIG. 7 is a flow chart representing the details of the training phase of FIG. 6, according to the present invention; and 
     FIG. 8 is a block diagram illustrating a specific example of a vectorization sub-module used in the training phase of FIG.  7 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following definitions and explanations provide background information pertaining to the technical field of the present invention, and are intended to facilitate the understanding of the present invention without limiting its scope: 
     Child (also daughter): From graph theory, a node pointed to by a path from a parent 
     DTD (Document Type Definition) defines the schema of semi-structured documents such as SGML(Standard Generalized Markup Language), HTML, or XML documents. 
     Element: “Tagged” text; text encoded with formatting or other information 
     Flat: Devoid of structure 
     Flatten: To remove structure, especially from an entity with implicit tree structure, in order to achieve a simple collection of leaves 
     HTML (Hypertext Markup Language): A standard language for attaching presentation and linking attributes to informational content within documents. During a document authoring stage, HTML “tags” are embedded within the informational content of the document. When the web document (or “HTML document”) is subsequently transmitted by a web server to a web browser, the tags are interpreted by the browser and used to parse and display the document. In addition to specifying how the web browser is to display the document, HTML tags can be used to create hyperlinks to other web documents. 
     Internet: A collection of interconnected public and private computer networks that are linked together with routers by a set of standards protocols to form a global, distributed network. 
     Leaf: Terminal node of a tree; a node with no child/daughter. 
     Node: A point or vertex in a graph. 
     Parent: A node (as in a graph) with a successor (child/daughter). 
     Root node: A node with no parent but typically with children. 
     Search engine: A remotely accessible World Wide Web tool that allows users to conduct keyword searches for information on the Internet. 
     Schema: Format or structure. It defines the structure and type of contents of constituent structures of, for example, a database, XML documents, etc. 
     Semi-structured: Implying a schema, either explicit or implicit but not conforming to a fixed schema. 
     Server: A software program or a computer that responds to requests from a web browser by returning (“serving”) web documents. 
     Tags: Codes (as in HTML or XML) that give instructions for formatting or action. 
     Tree: A hierarchical structure which is made up by nodes. Nodes are connected by edges from one node (parent) to another (child). A single node at apex of the tree is known as the root node, while the terminus of a path in the opposite direction is a leaf. 
     URL (Uniform Resource Locator): A unique address that fully specifies the location of a content object on the Internet. The general format of a URL is protocol://server-address/path/filename. 
     Vector: Broadly, a collection of objects or terms 
     Web browser: A software program that allows users to request and read hypertext documents. The browser gives some means of viewing the contents of web documents and of navigating from one document to another. 
     Web document or page: A collection of data available on the World Wide Web and identified by a URL. In the simplest, most common case, a web page is a file written in HTML and stored on a web server. It is possible for the server to generate pages dynamically in response to a request from the user. A web page can be in any format that the browser or a helper application can display. The format is transmitted as part of the headers of the response as a MIME type, e.g. “text/html”, “image/gif”. An HTML web page will typically refer to other web pages and Internet resources by including hypertext links. 
     Web Site: A database or other collection of inter-linked hypertext documents (“web documents” or “web pages”) and associated data entities, which is accessible via a computer network, and which forms part of a larger, distributed informational system such as the WWW. In general, a web site corresponds to a particular Internet domain name, and includes the content of a particular organization. Other types of web sites may include, for example, a hypertext database of a corporate “intranet” (i.e., an internal network which uses standard Internet protocols), or a site of a hypertext system that uses document retrieval protocols other than those of the WWW. 
     World Wide Web (WWW, also Web): An Internet client—server hypertext distributed information retrieval system. 
     XML: extensible Markup Language. A standard, semi-structured language used for Web documents. During a document authoring stage, XML “tags” are embedded within the informational content of the document. When the web document (or “XML document”) is subsequently transmitted by a web server to a web browser, the tags are interpreted by the browser and used to parse and display the document. In addition to specifying how the web browser is to display the document, XML tags can be used to create hyperlinks to other web documents. 
     FIG. 1 portrays the overall environment in which a classifier (classification system)  10  for semi-structured documents according to the present invention may be used. The classifier  10  includes a software or computer program product which is typically embedded within, or installed on a host server  15 . Alternatively, the classifier  10  can be saved on a suitable storage medium such as a diskette, a CD, a hard drive, or like devices. While the classifier  10  will be described in connection with the WWW, the classifier  10  can be used with a stand-alone database of terms that may have been derived from the WWW and/or other sources. 
     The cloud-like communication network  20  is comprised of communication lines and switches connecting servers such as servers  25 ,  27 , to gateways such as gateway  30 . The servers  25 ,  27  and the gateway  30  provide the communication access to the WWW Internet. Users, such as remote Internet users are represented by a variety of computers such as computers  35 ,  37 ,  39 , and can query the host server  15  for the desired information. 
     The host server  15  is connected to the network  20  via a communications link such as a telephone, cable, or satellite link. The servers  25 ,  27  can be connected via high speed Internet network lines  44 ,  46  to other computers and gateways. The servers  25 ,  27  provide access to stored information such as hypertext or web documents indicated generally at  50 ,  55 , and  60 . The hypertext documents  50 ,  55 ,  60  most likely include embedded hypertext link to other locally stored pages, and hypertext links  70 ,  72 ,  74 ,  76  to other webs sites or documents  55 ,  60  that are stored by various web servers such as the server  27 . 
     FIG. 2 illustrates a high-level architecture showing the classifier  10  used in the context of an Internet environment. The classifier  10  resides between the user and the semi-structured documents available for search on the WWW  20 . Documents judged to fall into a given category by the classifier will be made available to the user for their perusal and possible use. 
     As a specific example, a user uses a browser or a user interface (UI)  140  to enter a search query that is transmitted to a search service provider  100 . In turn, the search service provider  100 , accesses the classifier  10 . The classifier automatically searches the semi-structured documents on the WWW  20 . The search results will be sent to the user via the search service provider  100 . The search results may include a list of URLs and associated brief abstracts describing the nature of the resources found. 
     A conventional document classification system will now be described in more detail with further reference to FIG. 3. A document may be considered a collection of words or terms. From a classification standpoint, certain terms are considered to be significant and comprise by a vector that contains a component/dimension for each significant term. These are represented by bins W 1  through Wn in FIG.  3 . In this exemplary case, bin W 1  contains the number  12 , indicating that the term associated with bin W 1  appeared  12  times in the document. Similarly, the term associated with bin W 4  failed to appear in the document. The information available from the vector may be also represented by a histogram. 
     In a classification exercise, the classifier searches the text of a document. The number in each bin indicates the number of occurrences of a particular word in that document. This vector, because of its lack of structure, is indifferent to the meaning or context of the word. Reciting a previous example, the word “course” in the phases “golf course” and “of course” would not be segregated nor differentiated despite differences in meaning and context. 
     With reference to FIG. 4, the classifier  10  for semi-structured documents creates and relies on a structured vector model  200  that allows the classifier  10  to exploit structured information with text content embedded between the structural markups of XML documents. In FIG. 4, a hierarchical document is represented by a structured vector. The vector developed by the classifier  10  is divided into a tree of sub-vectors, labeled as  1 ,  2  and  3 . It should be noted that the tree structure is chosen for simplicity of exposition, though, in general, the documents can be modeled with directed graphs. The sub-vectors may be subdivided into a plurality of sub-sub-vectors, labeled  1 ,  2 ,  3 ,  4 , and  5 . Hierarchical levels continue downward until the leaves (childless elements) are reached. In XML documents, leaves correspond to pure text. By creating such a model, the classifier  10  is able to encode all of the structural and textual embedded in a document. 
     The vector of the structured vector model  200  of the present invention includes the sub-vectors of the children elements. By extension, a sub-vector includes the sub-sub-vectors of its children elements. 
     With a document  300  (FIG. 5) now parsed into the structured vector model  200 , the classifier is able to take into account the term frequency and distribution at each hierarchical level and, thus, achieve a contextual sensitivity heretofore unavailable. 
     The document  300  represented by the structured vector  200  of FIG. 4 may be alternatively represented by a vector using mathematical notation. Specifically, the document of FIG. 4 may be written as a compilation of all the sub-vectors: 
     
       
           e   d ( 0 , 0 )=&lt; e   d ( 1 , 0 ),  e   d ( 1 , 1 ), . . . ,  e   d ( 1 , m   d ( i )&gt;, 
       
     
     illustrating the fact that any vector includes the vectors of its child elements. 
     A path to an interior node e d (i,j), pd(i,j)=(e d ( 0 , 0 ). e d ( 1 ,P 1 ).e d ( 2 ,p 2 ) . . . e d (i,j)) is a sequence of structure nodes that must be visited to reach to the node e i,j  from the root. The path from the root to the node e d (i,j) is unique, as each node has only one parent node. Thus, p d (i,j) uniquely determines the location of the corresponding element&#39;s location in the document. 
     The path expression of each text term in the XML document can be computed by augmenting the tags of the structure elements it belongs to. For example, the term “course” in the education element in the following XML document yields “resume.education.course”, and the one in the hobby element yields “resume.hobby.course”. Terms in the same element share the same path, whereas the same term in different elements have different paths. Thus, it is possible to consider two terms as having different meanings while they have similar spelling. 
     FIG. 5 depicts the classifier  10  and the classification process based on the structured vector model  200  of the present invention. As illustrated, a document  300  is presented to the classifier  10  in order to determine its class. Referring back to FIG. 4, the structural and textual information in the document  300  is used to form the structure vector model  200  of the document. The classifier  10  is now able to characterize the term frequency and distribution of the document in question and compare it to that of the known classes of documents, labeled Class  1  through Class N, in FIG.  5 . 
     Compared to the conventional classifier (FIG. 2) the classifier  10  of the present invention extends beyond the uppermost level of the tree of FIG.  4 . The classifier  10  is able to gain added precision by continuing its analysis on the sub-vectors and sub-sub-vectors (and further sub-levels until it reaches the level that comprises only text terms) of the structured vector model  200 . In addition, the classifier  10  weights the significance of terms in the sub-vectors and sub-sub-vectors. 
     FIG. 6 is a representation of the training and testing phases used by the classifier  10 . In order for the classifier  10  to work optimally, it needs to be “trained” to recognize documents of a given class. As illustrated, the classifier  10  is given a large number of documents  350 . Some of these documents  352  have known class labels, and other documents  354  are without class labels. Those documents  352  with known class labels are fed into a training module  375  so that the classifier  10  learns the term frequency and distribution characteristics of the documents  352  of a given class. 
     Once these data are available, the documents  354  without class labels are entered in a testing module (also referred to herein as classification module)  385  which classifies the documents  354 . The classifier  10  attempts to classify the documents based on the data of the training documents. In particular, once the term frequency and distribution data for all levels of the vector are available and weightings have been assigned, the classifier  10  relies on a robust statistical model  200  to assign a class to the specific document  300  to be classified. The class  400  assigned is the one that maximizes the following a posteriori class probability Pr[c|d,F k ]:                  Pr   [   c             d     ,     F   k       ]     =       π                   (   c   )                     ∏                            t                 ε                   p   d                     (     i   ,   j     )       ,       t                 ε                 d     ⋂       F   k                     (       e   d                     (     i   ,   j     )       )                f                         (     c   ,       p   d                     (     i   ,   j     )       ,   t     )       n        (     d   ,       p   d                     (     i   ,   j     )       ,   t     )                           ∑                          c   ′          π                     (     c   ′     )                     ∏                            t                 ε                   p   d                     (     i   ·     )       ,       t                 ε                 d     ⋂       F   k                     (       e   d                     (     i   ,   j     )       )                f                         (     c   ,       p   d                     (     i   ,   j     )       ,   t     )       n        (     d   ,       p   d                     (     i   ,   j     )       ,   t     )                                                    
     where d is the document, π(c) is the prior distribution on the class c; c′ is a class in the set of documents; p d  is a path to a structure node e d  from the root; n is the number of occurrences of term t in p d , f is the maximum likelihood estimation; F k  is a set of selected terms; F is the Fisher index defined by the following equation, where c 1  and c 2  are children of the internal class c 0 , and μ is the average number of the occurrence of term t in class c:          F                   (   t   )       =       ∑                  (       μ                   (       c   1     ,   t     )       -     μ                   (       c   2     ,   t     )         )         2           c   1     ,     c   2                 ∑                        c          1        c                            ∑                  (       f                   (     t   ,   d   ,   c     )       -     μ                   (     c   ,   t     )         )         2         d                 ε                 c                                          
     FIG. 7 introduces the details of the training module  375  (FIG. 6) of the classifier  10  in the form of a flow chart. Within the training module  375  exist three elements: a vectorization sub-module  405 , a sorting sub-module  410 , and a modeling sub-module  415 . The flexibility of the training module  375  allows the relative locations of the sorting module  410  and the vectorization sub-module  405  to be interchanged. For illustration purpose, the vectorization sub-module  405  and its function will be described first. 
     With reference to FIG. 7, the training process begins by introducing a large number of documents  352  with known class labels into the classifier  10 . For each document  352 , the classifier  10  creates a structured vector, in particular, extracting textual and structural information from the document  352  and constructing a directed graph containing the information. This process is known as vectorization. 
     At each level or sublevel of the structured vector, the classifier  10  calculates the frequency and distribution of terms. The sorting sub-module  410  then sorts the documents  352  by classes, i.e., it classifies the documents  352 . At this point, the modeling sub-module  415  is invoked. This sub-module  415  is responsible for gathering the statistical information that will be used to classify documents  354  of unknown class. The statistics are calculated by combining all the documents of a given type together in a meaningful fashion. In particular, the modeling sub-module  415  combines the individual vectors in the class by adding them together and normalizing the result. Term frequencies may be normalized at any level from the uppermost (document level) to the lowest sub-vector. 
     Modeling may be likened to summation. Each word in the highest-level vector, as in FIG. 4, is assigned a specific bin in a particular order corresponding to the order of the lexicon of the class. The lexicon, thus, defines the order and kinds of words that are used to characterize a document  300 . From the vectors of all training documents  352  in a class the sum is calculated. The sum vector is then normalized by the number of documents  352 . The process is repeated until a statistical model  200  exists for each class of documents  352 . It is upon the basis of these statistical models  200  that classes  400  are assigned to documents  354  of previously undetermined classes. 
     FIG. 8 provides a specific example of the operation of the vectorization sub-module  405  of FIG. 7 operating on a document  352  with an XML format. For illustrative purposes, a sample document  352  corresponding to the structured vector of FIG. 8, appears below: 
     
       
         
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
               
               
             
           
               
                   
                   
               
             
             
               
                   
                       &lt;resume&gt; 
               
             
          
           
               
                   
                       &lt;experience&gt; 
               
             
          
           
               
                   
                           &lt;research assistant/&gt; 
               
               
                   
                 &lt;industry experience/&gt; 
               
             
          
           
               
                   
                       &lt;experience/&gt; 
               
               
                   
                 &lt;skills&gt; 
               
             
          
           
               
                   
                            &lt;technical/&gt; 
               
               
                   
                 &lt;other/&gt; 
               
             
          
           
               
                   
                       &lt;skills/&gt; 
               
               
                   
                 &lt;education&gt; 
               
             
          
           
               
                   
                            &lt;undergraduate/&gt; 
               
               
                   
                 &lt;graduate&gt; 
               
             
          
           
               
                   
                       &lt;education/&gt; 
               
             
          
           
               
                   
                      &lt;resume/&gt; 
               
               
                   
                   
               
             
          
         
       
     
     The vectorization sub-module  405  begins its function by parsing the document  352  to crate a parse tree of the document  352 . This parser can be an available semi-parser or an XML parser, such as “XML Parser for Java (http://www.alphaworks.ibm.com).” From the parsed tree, the vectorization sub-module  405  continues its function by creating a structured document vector shown in FIG.  8 . 
     The root of the tree is “Resume.” Sub-vectors are represented by Experience, Skills and Education. Each of these sub-vectors is underpinned by sub-sub-vectors (or grandchild vectors). As a specific example, Research Assistant and Industry Experience report to the sub-vector Experience. Sub-sub-vectors Technical and Other are children of the sub-vector Skills and grand-children of the vector Resume. 
     With the document  352  parsed and vectorized, the classifier  10  is now able to process the information contained in the structured vector. It begins by inspecting the terms in the document  352 . The first time a term is encountered at the sub or sub-sub-vector level the classifier  10  propagates the occurrence back to the parents of the term and all vectors in that path are updated. In particular, the count on the bin corresponding to that term is incremented. Counts are merged at the higher levels but at the lower levels the counts are separately maintained. The counts that are maintained on the lower levels are the keys to accurate and precise classification of semi-structured documents. 
     It is to be understood that the specific embodiments of the invention that have been described are merely illustrative of certain application of the principle of the present invention. Numerous modifications may be made to the classifier and associated method described herein without departing from the spirit and scope of the present invention. Moreover, while the present invention is described for illustration purpose only in relation to the WWW, it should be clear that the invention is applicable as well to databases and other tables with indexed entries.