Abstract:
A field-weighted search combines statistical information for each term across document fields in a suitably weighted fashion. Both field-specific term frequencies and field and document lengths are considered to obtain a field-weighted document weight for each query term. Each field-weighted document weight can then be combined in order to generate a field-weighted document score that is responsive to the overall query.

Description:
TECHNICAL FIELD 
   The invention relates generally to text document searching, and more particularly to field weighting in text document searching. 
   BACKGROUND 
   In a text document search, a user typically enters a query into a search engine. The search engine evaluates the query against a database of indexed documents and returns a ranked list of documents that best satisfy the query. A score, representing a measure of how well the document satisfies the query, is algorithmically generated by the search engine. Commonly-used scoring algorithms rely on splitting the query up into search terms and using statistical information about the occurrence of individual terms in the body of text documents to be searched. The documents are listed in rank order according to their corresponding scores so the user can see the best matching search results at the top of the search results list. 
   Many such scoring algorithms assume that each document is a single, undifferentiated string of text. The query of search terms is applied to the text string (or more accurately, to the statistics generated from the undifferentiated text string that represents each document). However, documents often have some internal structure (e.g., fields containing titles, section headings, metadata fields, etc.), and reducing such documents to an undifferentiated text string loses any searching benefit provided by such structural information. 
   Some existing approaches attempt to incorporate the internal structure of documents into a search by generating statistics for individual document fields and generating scores for individual fields. The score for an individual document is then computed as a weighted sum of scores for its fields. However, in such existing approaches, the weights applied to individual fields of different documents do not adequately consider the influence of document length, field lengths, and the possible combinations of term frequencies of different query terms in different fields on the overall score for a given document. 
   SUMMARY 
   Implementations described and claimed herein address the foregoing problems by combining statistical information for each term across document fields in a suitably weighted fashion. Both field-specific term frequencies and field lengths may be considered to obtain a field-weighted document weight for each query term. Each field-weighted document weight can then be combined in order to generate a field-weighted score that is responsive to the overall query. 
   In some implementations, articles of manufacture are provided as computer program products. One implementation of a computer program product provides a computer program storage medium readable by a computer system and encoding a computer program. Another implementation of a computer program product may be provided in a computer data signal embodied in a carrier wave by a computing system and encoding the computer program. 
   The computer program product encodes a computer program for executing on a computer system a computer process for determining a field-weighted score for a document having multiple fields relative to a query having a plurality of query terms. A field-weighted term frequency is computed for each query term based on field weights designated for individual fields in the document. A field-weighted document weight is computed for each query term based on the field-weighted term frequency for each field in the document. The field-weight score is computed based on the field-weighted document weights of the query terms. 
   In another implementation, a method is provided that determines a field-weighted score for a document having multiple fields relative to a query having a plurality of query terms. A field-weighted term frequency is computed for each query term based on field weights designated for individual fields in the document. A field-weighted document weight is computed for each query term based on the field-weighted term frequency for each field in the document. The field-weight score is computed based on the field-weighted document weights of the query terms. 
   In another implementation, a system for determining a field-weighted score for a document having multiple fields relative to a query having a plurality of query terms is provided. A field-weighted term frequency calculator computes a field-weighted term frequency for each query term based on field weights designated for individual fields in the document. A field-weighted document weight calculator computes a field-weighted document weight for each query term based on the field-weighted term frequency for each field in the document. A search engine computes the field-weighted score as a function of the field-weighted document weights of the query terms. 
   Other implementations are also described and recited herein. 

   
     BRIEF DESCRIPTIONS OF THE DRAWINGS 
       FIG. 1  illustrates an exemplary field-weighted search engine. 
       FIG. 2  illustrates an exemplary generation of a virtual document. 
       FIG. 3  illustrates an exemplary search engine employing a virtual document approach to provide a field-weighted search. 
       FIG. 4  illustrates an exemplary field weighting search engine employing a field weighting search algorithm to provide a field-weighted search. 
       FIG. 5  illustrates operations of an exemplary field-weighted searching process. 
       FIG. 6  illustrates operations of another exemplary field-weighted searching process. 
       FIG. 7  illustrates a system useful for implementing an embodiment of the present invention. 
   

   DETAILED DESCRIPTION 
     FIG. 1  illustrates an exemplary field-weighted search engine  100 . The search engine  100  receives a query  102  containing multiple query terms  104 ,  106 , and  108 . Each query term may include multiple component terms, such as if the query term is a phrase (e.g., the phrase “document management system” may be considered a single query term”). In addition, a query may include one or more operators, such as Boolean operators, constraints, etc., which are commonly supported by known search engines. 
   A plurality of documents, represented by documents  110 ,  112 ,  114 , and  116 , are available for searching. In practice, a search engine may search any number of documents and typically search collections containing large numbers (e.g., millions) of documents. An indexing module (not shown) generates individual document statistics (e.g.,  118 ,  120 ,  122 , and  124 ) for each document. The document statistics are stored in an index  126 . 
   The search engine  100  interrogates the index  126  to determine a search score  128  for each document based on the query  102  and the corresponding document statistics. Typically, the document scores  128  are then ranked in descending order to give the user a list of documents that are considered by the search algorithm to be most relevant to the query  102 . 
   In the illustrated system, the search engine  100  represents a field-weighted search engine, which considers the structure of a document in its search algorithm. For examples, a simple document structure may include fields, such as title, abstract, and body. Other exemplary types of fields may include without limitation headings, sections, conclusions, and metadata fields. 
   A field-weighted search may be expected to be more accurate when the different fields are expected to include the same general type of language, although a less constrained selection of fields may be effective as well. For example, fields such as title, abstract, and body might be expected to share common and important terms that are indicative of the relevance of the document to a given search. In contrast, an author field tends to include names (a different “type” of language) that are not expected to be contained within the title, abstract, and body. Accordingly, one implementation omits an author field from a field-weighted search for this reason. Author fields and other “different types” of fields are, nevertheless, eligible for consideration in a field-weighted search, in any combination. 
   In one implementation, aspects of field weighting may be introduced by way of a field weighting indexer (e.g., the virtual document index generator of  FIG. 3 ). In another implementation, fielding weighting features may be integrated into a field weighting search engine (e.g., the field weighting search engine of  FIG. 4 ). 
     FIG. 2  illustrates an exemplary generation of a virtual document  200  from a document  202  that includes multiple fields: title field  204 , abstract field  206 , and body field  208 . In this implementation, field weighting is introduced through the generation of a virtual documents  200  as influenced by field weight  210  (weight=5 in association with the title field  204 ), field weight  212  (weight=3 in association with the abstract field  206 ), and field weight  214  (weight=1 in association with the body field  208 ). A weight equaling zero may also be used, for example, to ignore the associated field in the search results. 
   In this implementation, each field of the document is replicated the number of times indicated by the field weight. The replicated field copies are concatenated to produce a field set (although other methods of combining the field copies into a field set may be employed). For example, the title  204  field is replicated five times to produce the title field set  216 , the abstract field  206  is replicated three times to produce the abstract field set  218 , and the body field  208  replicated once to product an abstract fields set  220 . 
   The three field sets are then concatenated together into the virtual document  200  (although other combinations are possible, such as mixing the field sets). As described with regard to  FIG. 3 , the virtual document may then be indexed to provide field-weighted virtual document statistics for the document  202 . These statistics may then be retrieved by a search engine to produce a field-weighted score for the document  202 . 
     FIG. 3  illustrates an exemplary search engine  300  employing a virtual document approach to provide a field-weighted search. The search engine  300  receives a query  302  containing multiple query terms. A plurality  304  of documents is available for searching. 
   A virtual document index generator  306  inputs each document and a set of field weights  308 . The index generator  306  generates a virtual document (not shown) from each input document, such as described with regard to  FIG. 2 . The index generator  306  then generates virtual document statistics  310  for each virtual document. The virtual document statistics  310  may include term statistics  312  (including without limitation the frequencies of each term in the document (i.e., “term frequency” in the document) and the locations of each term in the document) and a document length table  314  (which indicates the length of the document). 
   To describe the operations for an exemplary field-weighted search using the virtual document approach, the following terms and notations are introduced. (Certain terms are preceded by a parenthetical indicating that the terms are “field-weighted”. This description is meant to indicate that the document statistics of the virtual document implicitly include the influence of field weighting because the virtual document was constructed based on the field weights and the field lengths.): 
   Base Query Term Weight (w i )—a weight applied to the query term i (e.g., the definite article “the” may be given less weight than other more informative query terms “structured” or “document”) 
   (Field-weighted) Term Frequency (tf i,d )—the number of occurrences of term i in virtual document d 
   (Field-weighted) Document Length (dl d )—the length of virtual document d 
   (Field-weighted) document Weight (wd i )—a function of a base query weight w for each query term i, the Term Frequency (tf i,d ) in the virtual document d, the document length (did) of the virtual document d, and possibly other document-specific information (dl d ) (i.e., wd i (w i ,tf i,d ,dl d ,θ d )); this function may be linear or non-linear 
   Document Score (sc d )—combines the document weights of all the terms of a query (indexed 1, . . . , V) into a single document score (e.g., sc d (wd 1 ,wd 2 , . . . , wd v )); may be a linear combination or a non-linear combination 
   The search engine  300  then applies its search algorithm to generate a field-weighted document weight for each query term from the virtual document statistics. There exist a variety of methods for computing document weights and most can be used to compute the (field-weighted) document weight given the parameters w i , tf i,d , dl d , and θ d  based on the virtual document. The search engine  300  then combines the field-weighted term weights for each document into a field-weighted document score  316 . Typically, the document scores  316  for evaluated documents are then ranked in descending order to give the user a list of documents that are considered by the search algorithm to be most relevant to the query  302 . 
   The virtual document approach described with regard to  FIGS. 2 and 3  represents an effective implementation of a field-weighted search. Another implementation, discussed with regard to  FIG. 4 , employs document statistics for each document without resorting to generation of a virtual document. The document statistics employed in the implementation illustrated in  FIG. 4  includes field-specific information, such as the field location of each term (e.g., which field), field-specific term frequencies, and field lengths. 
     FIG. 4  illustrates an exemplary field weighting search engine  400  employing a field weighting search algorithm to provide a field-weighted search. To describe the operations for an exemplary field-weighted search, the following terms and notations are introduced: 
   Base Query Term Weight (w i )—a weight applied to the query term i 
   Term Frequency (tf i,d )—the number of occurrences of term i in document d 
   Field-specific Term Frequency (tf i,d,f )—the number of occurrences of a term i in field f of document d 
   Document Length (dl d )—the length of document d 
   Field Length (dl d,f )—the length of field f in document d 
   Document Weight (wd i )—a function of a base query weight w for each query term i, the Term Frequency (tf i,d ) in the document d, the document length (dl d ) of the document d, and possibly other document-specific information (θ d ) (i.e., wd i (w i ,tf i,d ,dl d ,θ d )) 
   Field-weighted Term Frequency (ntf i,d )—a combination of field weights and field-specific term frequencies tf for a term i and a document d 
   Field-weighted Document Length (ndl d )—a combination of field-weights and field lengths dl for a document d 
   Field-weighted Document Weight (fwd i )—a function of a base query weight w for each query term, the Field-weighted Term Frequency (ntf i,d ) in the document d, the Field-weighted document length (ndl d ) of the document d, and possibly other document-specific information (θ d ) (i.e., fwd i (w i ,ntf i,d ,ndl d ,θ d )); this function may be linear or non-linear 
   Field-weighted Document Score (fsc d )—combines the weights of all the field-weighted document weights of all the terms of a query (indexed 1, . . . , V) into a single document score (i.e., fsc d (fwd 1 ,fwd 2 , . . . , fwd v )); may be a linear combination or a non-linear combination 
   An index containing exemplary document statistics  402  are input to the field weighting search engine  400 . The exemplary document statistics  402  include without limitation a term statistics  404  (including without limitation the frequencies of each term in each field (i.e., “field-specific term frequency”) and the locations of each term in each field) and a document length table  406  (which indicates the length of each field). It should be understood that the document statistics  402  may include additional statistics, such as the locations of each term in the document, and the document length table  406  may include additional information, such the length of the overall document. 
   A multi-term query  408  is input to the field weighting search engine  400 . A field-weighted term frequency calculator  410  inputs the query terms from the query  408  and field weights  412  The field-weighted term frequency calculator  410  also retrieves appropriate document statistics  402  (e.g., for each query terms). Based on these inputs, the field-weighted term frequency calculator  410  computes a field-weighted term frequency for each query term for each document. In one implementation, the field-weighted term frequency for a query term i and a document d is computed using the equation 
                   ntf     i   ,   d       :=       ∑     f   ∈     document   ⁢           ⁢   fields         ⁢       m   f     ⁢     tf     i   ,   d   ,   f                   (   1   )               
where m f  represents a field weight for field f and tf i,d,f  represents a field-specific term frequency for the query term i, a field f, and the document d. However, it should be understood that the algorithm of Equation (1) is merely exemplary and that other algorithms may alternatively be employed.
 
   A field-weighted document length calculator  414  inputs the field weights  412  and appropriate document statistics (e.g., field lengths). Based on these inputs, the field-weighted document length calculator  414  computes a field-weighted document length for each document. In one implementation, the field-weighted document length for a document d is computed using the equation 
                   ndl   d     :=       ∑     f   ∈     document   ⁢           ⁢   fields         ⁢       m   f     ⁢     dl     d   ,   f                   (   2   )               
where m f  represents a field weight for field f and dl d,f  represents a field length for a field f and the document d. However, it should be understood that the algorithm of Equation (2) is merely exemplary and that other algorithms may alternatively be employed.
 
   A field-weighted document weight calculator  416  computes a field-weighted document weight fwd i  for each term i in each document d as a function of the base query term weight, the field-weighted term frequency, the field-weighted document length, and possibly other document-specific information (e.g., nfwd i (w i ,ntf i,d ,ndl d ,θ d )). There exist a variety of methods for computing document weights and most can be used to compute the field-weighted document weight given these parameters. A document score calculator  418  computes a field-weighted document score  420  for each document searched. Typically, the document scores  420  for evaluated documents are then ranked in descending order to give the user a list of documents that are considered by the search algorithm to be most relevant to the query  408 . 
     FIG. 5  illustrates operations  500  of an exemplary field-weighted searching process using a virtual document approach. A field weight operation  502  determines the weights associated with each field of a set of searchable documents. Some documents may have different fields than other documents, and some fields existing in some documents in the set may not be weighted or may have weights set to zero. 
   A generating operation  504  generates field sets for each document based on the field weights and the identification of the fields of each document. Another generation operation  506  combine the field sets of each document to product an individual virtual document corresponding to each document. An indexing operation  508  analyzes each virtual document and generates virtual document statistics, which implicitly reflect the field-based influences of the document set. 
   An extraction operation  510  parses the query to determine the query terms. Base query term weights (e.g., w i ) are determined for each query term in a determination operation  512 . A computing operation  514  computes for each virtual document the field-weighted document weights for each term from the field-weighted term frequency from the virtual document statistics. The computing operation  514  then computes a field-weighted document score based on the virtual document, which is associated with the original document. In some implementations, the document scores are ranked in ranking operation  516  and displayed in a user interface in the descending order of document score, reflecting each document&#39;s anticipated relevance. 
     FIG. 6  illustrates operations  600  of another exemplary field-weighted searching process. An extraction operation  602  parses the query to determine the query terms. Base query term weights (e.g., w i ) are determined for each query term in a determination operation  604 . 
   A document index containing document statistics is retrieved by retrieval operation  606 . Another retrieval operation  608  retrieves from the document index field-specific term frequencies and field lengths for each field of each document, based on the field weights. A calculation operation  610  computes the field-weighted term frequency and field-weighted document length for each document based on the field-specific term frequencies and field lengths. Another calculation operation  612  computes a field-weighted document weight for each term based on the field-weighted term frequency and field-weighted document length for each document. A computation operation  614  then computes a field-weighted document score based on the virtual document, which is associated with the original document. In some implementations, the document scores are ranked in ranking operation  616  and displayed in a user interface in the descending order of document score, reflecting each document&#39;s anticipated relevance. 
   Document weights w i  can also be modified by the number of fields in the document in which the term i occurs, represented by the multiple field factor ff i,d . In one implementation, a suitable function may take the form: 
               nfwd   i     ⁡     (       w   i     ,     ntf     i   ,   d       ,     ndl   d     ,     ff   i     ,     θ   d       )       :=         fwd   i     ⁡     (       w   i     ,     ntf     i   ,   d       ,     ndl   d     ,     θ   d       )       ·       ff   i         k   f     +     ff   i                 
where k f  represents a scalar constant that controls the extent of the multiple field effect.
 
   Document score fsc d  can also be modified based on detection of multiple query terms in a given field, represented by the multiple term factor bf d . In one implementation, a suitable function may be implemented as follow: 
             bf   d     :=         max     f   ∈     document   ⁢           ⁢   fields         ⁢     {       ∑     i   ❘       tf     i   ,   d   ,   f       ≠   0         ⁢     fwd   i       }           ∑       i   =   1     ,   …   ⁢           ,   V       ⁢     fwd   i               
Here bf d  represents a quantity between 0 and 1, which may be used to multiply the document score or may be used in a factor to modify the document score. In one implementation, the document score may be computed as:
   nfsc   d   :=[k   m   bf   d +(1− k   m )] fsc ( nfwd   1   ,nfwd   2   , . . . , nfwd   v ) 
   One specific implementation is based on the BM25 ranking formula (see e.g., Robertson, S. E., Walker, S., Beaulieu, M. M., Gatford, M., Payne, A. (1995): Okapi at TREC-4, in NIST Special Publication 500-236: The Fourth Text Retrieval Conference (TREC-4): 73-96). In such a context, the field-weighted document weights fwd i  may be determined as follows: 
             fwd   i     :=           (       k   1     +   1     )     ⁢     ntf     i   ,   d               k   1     ⁡     (       (     1   -   b     )     +     b   ⁢       ndl   d     avndl         )       +     ntf     i   ,   d           ⁢     w   i             
where ndl d  represents the field-weighted document length, avndl is the average field-weighted document length across the collection of documents, and k l  and b are free parameters. The basic query term weight w i  in the BM25 function is normally calculated as follows:
 
             w   i     =     log   ⁢       N   -     df   i     +   0.5         df   i     +   0.5               
where df i  is the number of documents in which query term i occurs and N is the total number of documents in the collection. The field-weighted document score fsc d  may then be obtained by adding the field-weighted document weights:
 
             fsc   d     =       ∑   i     ⁢     fwd   i             
It should be understood that other scoring algorithms, either based on BM25 or otherwise, may also be employed.
 
   Given the BM25 implementation described above, the parameters k l  and b may be optimized once for the non-field-weighted configuration (e.g., all field weights equaling 1 represents an exemplary non-field-weighted configuration) for a given document collection and then be applied to all other combinations of fields weights for that collection. For example, assuming that k l * and b* represent the optimal values for the non-field weighted case, k l  and b may be optimized of a field-weighted case by:
         (1) Calculating the average term frequency over all terms and all documents in the non-field-weighted configuration, atf unweighted .   (2) For a particular combination of field weights, calculating the average term frequency, atf weighted  (e.g., an average of the field-weighted term frequencies of all terms and all documents in the field-weighted configuration).   (3) Calculating the optimal b for the field weight combination as b=b*.   (4) Calculating the optimal k l  for the field weight combination as       

   
     
       
         
           
             k 
             1 
           
           = 
           
             
               k 
               1 
               * 
             
             ⁢ 
             
               
                 
                   atf 
                   weighted 
                 
                 
                   atf 
                   unweighted 
                 
               
               . 
             
           
         
       
     
   
   The exemplary hardware and operating environment of  FIG. 7  for implementing the invention includes a general purpose computing device in the form of a computer  20 , including a processing unit  21 , a system memory  22 , and a system bus  23  that operatively couples various system components include the system memory to the processing unit  21 . There may be only one or there may be more than one processing unit  21 , such that the processor of computer  20  comprises a single central-processing unit (CPU), or a plurality of processing units, commonly referred to as a parallel processing environment. The computer  20  may be a conventional computer, a distributed computer, or any other type of computer; the invention is not so limited. 
   The system bus  23  may be any of several types of bus structures including a memory bus or memory controller, a peripheral bus, a switched fabric, point-to-point connections, and a local bus using any of a variety of bus architectures. The system memory may also be referred to as simply the memory, and includes read only memory (ROM)  24  and random access memory (RAM)  25 . A basic input/output system (BIOS)  26 , containing the basic routines that help to transfer information between elements within the computer  20 , such as during start-up, is stored in ROM  24 . The computer  20  further includes a hard disk drive  27  for reading from and writing to a hard disk, not shown, a magnetic disk drive  28  for reading from or writing to a removable magnetic disk  29 , and an optical disk drive  30  for reading from or writing to a removable optical disk  31  such as a CD ROM or other optical media. 
   The hard disk drive  27 , magnetic disk drive  28 , and optical disk drive  30  are connected to the system bus  23  by a hard disk drive interface  32 , a magnetic disk drive interface  33 , and an optical disk drive interface  34 , respectively. The drives and their associated computer-readable media provide nonvolatile storage of computer-readable instructions, data structures, program modules and other data for the computer  20 . It should be appreciated by those skilled in the art that any type of computer-readable media which can store data that is accessible by a computer, such as magnetic cassettes, flash memory cards, digital video disks, random access memories (RAMs), read only memories (ROMs), and the like, may be used in the exemplary operating environment. 
   A number of program modules may be stored on the hard disk, magnetic disk  29 , optical disk  31 , ROM  24 , or RAM  25 , including an operating system  35 , one or more application programs  36 , other program modules  37 , and program data  38 . A user may enter commands and information into the personal computer  20  through input devices such as a keyboard  40  and pointing device  42 . Other input devices (not shown) may include a microphone, joystick, game pad, satellite  11  dish, scanner, or the like. These and other input devices are often connected to the processing unit  21  through a serial port interface  46  that is coupled to the system bus, but may be connected by other interfaces, such as a parallel port, game port, or a universal serial bus (USB). A monitor  47  or other type of display device is also connected to the system bus  23  via an interface, such as a video adapter  48 . In addition to the monitor, computers typically include other peripheral output devices (not shown), such as speakers and printers. 
   The computer  20  may operate in a networked environment using logical connections to one or more remote computers, such as remote computer  49 . These logical connections are achieved by a communication device coupled to or a part of the computer  20 ; the invention is not limited to a particular type of communications device. The remote computer  49  may be another computer, a server, a router, a network PC, a client, a peer device or other common network node, and typically includes many or all of the elements described above relative to the computer  20 , although only a memory storage device  50  has been illustrated in  FIG. 7 . The logical connections depicted in  FIG. 7  include a local-area network (LAN)  51  and a wide-area network (WAN)  52 . Such networking environments are commonplace in office networks, enterprise-wide computer networks, intranets and the Internet, which are all types of networks. 
   When used in a LAN-networking environment, the computer  20  is connected to the local network  51  through a network interface or adapter  53 , which is one type of communications device. When used in a WAN-networking environment, the computer  20  typically includes a modem  54 , a network adapter, a type of communications device, or any other type of communications device for establishing communications over the wide area network  52 . The modem  54 , which may be internal or external, is connected to the system bus  23  via the serial port interface  46 . In a networked environment, program modules depicted relative to the personal computer  20 , or portions thereof, may be stored in the remote memory storage device. It is appreciated that the network connections shown are exemplary and other means of and communications devices for establishing a communications link between the computers may be used. 
   In an exemplary implementation, a search engine, a virtual document index generator, a field-weighted term frequency calculator, a field-weighted document length calculator, and other modules may be incorporated as part of the operating system  35 , application programs  36 , or other program modules  37 . Document statistics, search scores, and other data may be stored as program data  38 . 
   The embodiments of the invention described herein are implemented as logical steps in one or more computer systems. The logical operations of the present invention are implemented (1) as a sequence of processor-implemented steps executing in one or more computer systems and (2) as interconnected machine modules within one or more computer systems. The implementation is a matter of choice, dependent on the performance requirements of the computer system implementing the invention. Accordingly, the logical operations making up the embodiments of the invention described herein are referred to variously as operations, steps, objects, or modules. 
   The above specification, examples and data provide a complete description of the structure and use of exemplary embodiments of the invention. Since many embodiments of the invention can be made without departing from the spirit and scope of the invention, the invention resides in the claims hereinafter appended.