Patent Publication Number: US-6665668-B1

Title: Document retrieval method and system and computer readable storage medium

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to document retrieval techniques for retrieving a registered document in accordance with an input query expression and displaying information of the retrieved document. 
     2. Description of the Related Art 
     In recent years, the number of electronic documents formed by a word processor and the like is increasing, and it is expected that the number of such documents increases in the future. A database used for the document retrieval is also becoming large in scale. Therefore, the set of documents, which is a search result obtained by the document retrieval is also becoming large. It is difficult for a user to find a really desired document from them. 
     In order to solve this problem, there is a ranking technique as the related art. The ranking technique is specifically described in “Ranking Algorithms”, by Donna Harman, Information Retrieval, pp. 363-392. This technique is hereinafter called “Related Art 1”. Related Art 1 provides a technique of calculating a factor which shows the possibility of being similar to the contents of a query expression (sentence, document, or a sequence of words) designated by the user. An example of the contents will be described with reference to FIG.  2 . 
     A retrieval (or search) is realized by a simple vector operation. Each element of this vector corresponds to words after excluding the overlapped words from all words appearing in the database (however, stop words and the like are excluded). In the example shown in FIG. 2, the elements are constituted of (factors, information, help, human, operation, retrieval, systems). “1” is set at the corresponding position if the query expression contains the element, and “0” is set at the corresponding position if the query expression does not contain the element. In this manner, vector Q 0  of the query expression can be formed. That is, vector Q 0  (1, 1, 0, 1, 0, 1, 1) is formed for query expression “human factors in information retrieval systems”. 
     A vector of document is similarly formed for each document in the database. Vector V 1  (1, 1, 0, 1, 0, 1, 0) is formed for Document 1 containing “factors”, “information”, “human” and “retrieval”. Vector V 2  (1, 0, 1, 1, 0, 0, 1) is formed for Document 2 containing “factors”, “help”, “human” and “systems”. Vector V 3  (1, 0, 0, 0, 1, 0, 1) is formed for Document 3 containing “factors”, “operation” and “systems”. 
     A score used for ranking is calculated from vector operation Vi·Q 0  between vector Q 0  of the query expression and vector Vi (i=1, 2, 3) of each document. The calculation results are score “4” for Document 1, score “3” for Document 2, and score “2” for Document 3. Each score represents the similarity to the query expression judged by the system. The document having the higher score has the higher possibility of being similar to the contents of the query expression. 
     Instead of expressing the element of the vector as “1” or “0”, the element may be expressed by the weight of word (calculated from the location frequency of the word, the location deviation of the word in the document database, or the like). For example, if the weight of “factors” is “2”, the weight of “information” is “3”, the weight of “human” is “5” and the weight of “retrieval” is “3”, then vector V′ 1  (2, 3, 0, 5, 0, 3, 0) can be formed for Document 1. Similarly, if the weight of “factors” is “2”, the weight of “help” is “4”, the weight of “human” is “5” and the weight of “systems” is “1”, then vector V′ 2  (2, 0, 4, 5, 0, 0, 1) can be formed for Document 2. Furthermore, if the weight of “factors” is “2”, the weight of “operation” is “2” and the weight of “system” is “1”, then vector V′ 3  (2, 0, 0, 0, 2, 0, 1) can be formed for Document 3. 
     The score of each document can be calculated from vector operation V′ 1 ·Q 0  between vector V′ 1  and query expression vector Q 0 . The calculation results are score “13” for Document 1, score “8” for Document 2 and score “3” for Document 3. Each score represents the similarity to the query expression, which is judged by the system in consideration of the weight of word, i.e., the importance degree of word. The document having the higher score has the higher possibility of being similar to the contents of the query expression. That is, the search result shows that Document 1 has the highest possibility of being similar to the contents of the query expression. 
     In Related Art 1, the factor which shows the possibility of being similar to the contents of the query expression is calculated. By browsing the documents in accordance with this factor, the desired document can be searched at high speed from the large-scale document database. However, whether or not the search result document is really the desired document is judged by the user by actually reading the contents of the document. As the technique of supporting the instant judgement of whether or not the document obtained as the search result is really the desired document, there is the document highlighting technology which is hereinafter called “Related Art 2”. 
     In Related Art 2, when the contents of the document obtained as the search result is displayed, a portion containing a character string of the query expression designated by the user is displayed in a display format (hereinafter called “a highlight”) different from that of other character string portions. The display format includes color, size, font, style (bold or roman) and the like. By displaying the portion containing the character string of the query expression in the display format different from that of other character string portions, it is possible to recognize at once the position containing the word. As a result, whether or not the document is the desired document can be judged faster than reading the document from the start thereof. 
     A word is often used as the element of the vector used by the ranking technique of Related Art 1. In a language such as English language in which each word is written in a delimiting manner, all words excepting stop words (such as “in” and “the”) are used as the vector elements. In a language such as Japanese language in which each word is not written in a delimiting manner, a character string obtained by dividing the different character types, consecutive n characters (“n” is a predetermined integer of “1” or larger), a word derived with reference to a dictionary or the like, and so forth are used as the vector elements. As a result, if a document or a long sentence is designated as the query expression-to execute the retrieval and the document obtained as the search result is displayed in accordance with the highlighting technology shown in Related Art 2, the number of character strings to be highlighted becomes large. Thereby, there is a problem that the important portion becomes difficult to be found. 
     This problem will be described with reference to FIG. 3 by taking a newspaper article database as an example. In this example, a newspaper article document regarding the stadium invitation for world cup of football is designated as the query expression to execute the retrieval. 
     First, character strings used for the retrieval are extracted from document “Football match stadiums for W-Cup will be determined next month, selection right attributed to Association. The organizing arrangement committee for the 2002 football world cup under the joint auspices of Japan and Korea opened on 29th, a governor/mayor meeting is held by calling special directors from fifteen local self-governing bodies which are candidates for organizing the stadium. For the number of stadiums in Japan, Federation International de Football Association (FIFA) . . . ” which is designated as the query expression. In the example shown in FIG. 3, nouns, katakana characters and gerunds, which are extracted by referring to a dictionary and the like, are extracted as the character string used for the retrieval. As a result, “football, W-Cup, match, stadium, next, month, determined, selection, right, Association, Japan, Korea, joint, auspices, world, cup, organizing, arrangement, committee, place, candidates, . . . ” are extracted from the search expression. By using these character strings, the document retrieval is executed. The factors which shows the possibility of being similar to the contents of the query expression are calculated and output together with a list of the documents. In this state, the user browses the documents staring from the document which has the highest possibility of being similar to the contents of the query expression, i.e., the document having the highest score, and confirms whether or not the document is the really desired document. If the highlighting technology such as Related Art 2 is incorporated, the position containing the character string of the query expression can be confirmed at once, so that whether or not the document is the desired document can be judged faster than reading the document from the start thereof. However, as shown in FIG. 3, if the document or the long sentence is designated to execute the retrieval and the documents obtained as the result are displayed, since the number of character strings used for the retrieval is large, there is a large number of highlighted portions (in the example shown in FIG. 3, large font sizes, roman type, and emphasis). It becomes therefore rather difficult to find the important portions. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to realize a document information display function which allows a user to easily judge whether or not a retrieved document is a desired document. 
     In order to solve the above-described problems, the invention comprises the following steps. 
     That is, it comprises: a document retrieval step of calculating as a similarity by a predetermined calculation method a degree of similarity between contents of a query expression designated by a user and contents of a text in a text database storing document information as character code data; and a document display step of selecting important information from information used for calculating the similarity in the document retrieval step to display the selected important information. 
     The principle of the present invention using the above-mentioned document retrieval method will be described in the following. When a user designates a sentence or a document as the query expression in retrieving a document, the document retrieval step mentioned above is executed to calculate the similarity by the predetermined calculation method. Here, the similarity is defined as the degree of similarity between the contents of the query expression designated by the user and the contents of each text in the text database. An example of the process contents of the document retrieval step will be described. First, the predetermined character strings are extracted from the designated query expression (hereinafter called “a query expression document”). As this character strings, words are used for a language such as English language in which each word is written in a delimiting manner, and for other languages, character strings obtained by dividing the different character types, consecutive character string consisting of n characters (“n” is the predetermined integer of 1 or larger), words derived with reference to a dictionary and the like, and so forth are used. If query expression document “Football match stadiums for W-Cup will be determined next month, selection right attributed to Association. The organizing arrangement committee for the 2002 football world cup under the joint auspices of Japan and Korea opened on 29th, a governor/mayor meeting by calling special directors from fifteen local self-governing bodies which are candidates for organizing the stadium. For the number of stadiums in Japan, Federation International de Football Association (FIFA) . . . ” is designated, and if nouns, katakana characters and gerunds are extracted as the character strings by referring to a dictionary and the like, then as shown in FIG. 3, “football, W-Cup, match, stadium, next, month, determined, selection, right, Association, Japan, Korea, joint, auspices, world, cup, organizing, arrangement, committee, place, candidates, . . . ” is extracted. The location information of these character strings in the text database is extracted. Although the location information changes with the retrieval method to be used, the serial number of the document which the character strings appears, the position of each location, the number of locations and the like are used. In Related Art 1, the serial number of the document which contains the character strings necessary for forming the vector of the document and the number of locations thereof are used. Next, the weight of each character string is calculated from this location information by using the predetermined calculation method. Although the calculation method changes with the retrieval method to be used, this weight is calculated by using the location frequency of each character string, the location deviation of each character string in the document database or the like. The weight calculated from the location deviation is generally an IDF (Inverse Document Frequency) described in “Ranking Algorithms” by Donna Harman, Information Retrieval, pp.363-392. The IDF is the weight which is proposed based on the concept that there is the high possibility that the character string contained in many documents is the stop word and has the low importance degree. The similarity is calculated in the predetermined calculation method using the location information and the weight. Although the calculation method changes with the retrieval method to be used, the simple vector operation used in Related Art 1 illustrated in FIG. 2 may be used for this calculation. The calculated similarities are displayed as the search result list. 
     In response to the display request of the document selected in the search result list, the document display step mentioned above is executed to select the important information from the information used for calculating the similarity in the document retrieval step and display the selected important information. An example of the process contents of the document display step will be described. First, the character strings and their weights extracted from the query expression in the document retrieval step are arranged in the descending order of the weight. The upper m (“m” is a predetermined integer of 1 or larger) character strings of the arranged character strings are extracted. The value of “m” may be automatically set to the proper value by the system itself, or may be set by the user beforehand. Alternatively, the user may set and adjust the value to the appropriate value interactively every document display. An example of the extracted character strings is shown in FIG.  4 . In the example shown in FIG. 4, “m” is set to “4”. The upper four character strings arranged in the descending order of the weight are extracted. As a result, “W-Cup”, “football”, “world cup” and “FIFA” are extracted. Next, the display format of the portion containing the extracted character string in the document designated to be displayed by the user (hereinafter called “a selected document to display”) is changed, and the selected document to display is displayed. As shown in FIG. 4, the display format of the portions containing the extracted “W-Cup”, “football”, “world cup” and “FIFA” is changed (in the example shown in FIG. 4, in the large size, roman bold fonts), and the document is displayed. The user can therefore confirm the important portions in the document at once. In this process example, the important information mentioned above means the highlighting information on the important character strings, and the important character string means the upper m (“m” is a predetermined integer of 1 or larger) character strings of the character strings arranged in the descending order of the weight. 
     As described above, in this method, the character strings which affect the factor which shows the possibility of being similar to the contents of the query expression, e.g., the predetermined number of character strings as counted from the highest weight are selected. As the information on these character strings, the document is displayed after changing the display format of the portions containing the character strings. As a result, since only the information on the important character strings of the character strings used for the retrieval is displayed, the user can confirm the important portions in the document at once, and can quickly judge whether or not the document is the desired document. Therefore, the quality of the user interface for browsing the search result document can be improved. 
     A program realizing the above-described function or a storage medium storing such a program may be used in order to achieve the above-described object. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram showing the structure of a first embodiment of the invention. 
     FIG. 2 is a diagram illustrating the process contents in Related Art 1. 
     FIG. 3 is a diagram illustrating a problem associated with Related Art 2. 
     FIG. 4 is a diagram illustrating the contents of a document display process of the invention. 
     FIG. 5 is a problem analysis diagram (PAD) showing the process contents of a document registration control program  112 . 
     FIG. 6 is a PAD showing the process contents of a document retrieval control program  116 . 
     FIG. 7 is a PAD showing the process contents of a document display control program  123 . 
     FIG. 8 is a diagram illustrating an example of forming a location information file  108  and a weight file  109 . 
     FIG. 9 is a diagram illustrating the contents of programs under the control of a document retrieval control program  116   a  in a second embodiment of the invention. 
     FIG. 10 is a diagram illustrating the contents of programs under the control of a document display control program  123   a  in the second embodiment of the invention. 
     FIG. 11 is a PAD showing the process contents of the document retrieval control program  116   a.    
     FIG. 12 is a PAD showing the process contents of the document display control program  123   a.    
     FIG. 13 is a diagram showing an example of a screen displaying the query term editing and the selection of the selected string to highlight. 
     FIG. 14 is a diagram illustrating the contents of a document display process in the second embodiment of the invention. 
     FIG. 15 is a diagram illustrating the contents of programs under the control of a document display control program  123   b  in a third embodiment of the invention. 
     FIG. 16 is a PAD illustrating the process contents of the document display control program  123   b.    
     FIG. 17 is a diagram illustrating the contents of a location information acquiring process and a similarity-every-character-string calculating process in the third embodiment of the invention. 
     FIG. 18 is a diagram illustrating the contents of a display format change and character string extracting process, a display format changing process, and a document display process in the third embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     A first embodiment of the invention will be described with reference to FIG.  1 . 
     A document retrieval system of the invention has a display  100 , a keyboard  101 , a central processing unit (CPU)  102 , a magnetic disk drive  106 , a floppy disk drive (FDD)  104 , a main memory  110  and a bus  103  interconnecting these units. The magnetic disk drive  106  is one of the secondary storage units and stores texts  107 , a location information file  108  and a weight file  109 . Information stored in a floppy disk  105  can be accessed from the FDD  104 . 
     The main memory  110  stores a system control program  111 , a document registration control program  112 , a text registration program  113 , a location information file generation program  114 , a weight file generation program  115 , a document retrieval control program  116 , a query parsing program  117 , a location information and weight acquisition program  119 , a weight storage program  120 , a similarity calculation program  121 , a search result list display program  122 , a document display control program  123 , a query term arrangement program  124 , a string-for-changing-display-format extraction program  125 , a display format change program  126  and a document display program  127 , and retains a work area  128 . A query term extraction program  118  is contained in the query parsing program  117 . These programs may be stored in a computer readable storage medium such as the magnetic disk  106  and the floppy disk  105 . 
     The system control program  111  is activated in response to a command from the keyboard  101 . The document registration control program  112  is activated by the system contort program  111  in response to a document registration command from the keyboard  101 , and controls the text registration program  113 , the location information file generation program  114  and the weight file generation program  115 . The document retrieval control program  116  is activated by the system contort program  111  in response to a document retrieval command from the keyboard  101 , and controls the query parsing program  117 , the location information and weight acquisition program  119 , the weight storage program  120 , the similarity calculation program  121  and the search result list display program  122 . The document display control program  123  is activated by the system contort program  111  in response to a document display command from the keyboard  101 , and controls the query term arrangement program  124 , the string-for-changing-display-format extraction program  125 , the display format change program  126  and the document display program  127 . 
     Next, the summary of the process contents of the embodiment will be described. 
     In registering a document, in response to the document registration command from the keyboard  101 , the system control program  111  activates the document registration control program  112 . The document registration control program  112  executes a series of document registration processes by using the text registration program  113 , the location information file generation program  114  and the file generation program  115 . The document registration process contents, i.e., the process contents of the document registration program  112  are illustrated in a PAD (Problem Analysis Diagram) of FIG.  5 . As shown in FIG. 5, at Step  600 , the document registration control program  112  activates the text registration program  113  to read the text data of a document to be registered, from the floppy disk  105  inserted into the FDD  104 , and to store the read text data as a text  107  in the magnetic disk  106 . The text data may be input not only from the floppy disk  105  but also from another system by using a communication line and a CD-ROM (not shown in FIG.  1 ). Next, at Step  601 , the location information file generation program  114  is activated to extract the predetermined character strings and their location information from the text data read at Step  600 , and to store the extracted character strings and their location information in the magnetic disk  106  as the location information file  108 . As the character strings to be extracted, words are used for a language such as English language in which each word is written in a delimiting manner, and for other languages, character strings obtained by dividing the different character types, consecutive n characters (“n” is a predetermined integer of 1 or larger), words derived with reference to a dictionary and the like, and so forth are used. Although the location information changes with the retrieval method to be used, the location information necessary for the retrieval includes the serial number of the document which contains the character strings, the position of each location thereof, the number of locations thereof and the like. In Related Art 1, the serial number of the document which contains the character strings necessary for forming a vector of a document and the number of locations thereof are used. Lastly, at Step  602 , the weight file generation program  115  is activated to calculate the weight of each character string extracted at Step  601  by using a predetermined calculation method, and to store it in the magnetic disk  106  as the weight file  109 . Although the calculation method changes with the retrieval method to be used, this weight is calculated by using the location frequency of each character string, the location deviation of each character string in the document database, or the like (generally by using the IDF (Inverted Document Frequency) described in “Ranking Algorithms” by Donna Harman, Information Retrieval, pp. 363-392). Thereafter, the document registration process is terminated. 
     In retrieving a document, the system control program  111  activates the document retrieval control program  116  in response to the document retrieval command from the keyboard  101 . The document retrieval control program  116  controls a series of the document retrieval processes by using the query parsing program  117 , the location information and weight acquisition program  119 , the weight storage program  120 , the similarity calculation program  121 , and the search result list display program  122 . 
     For a preparatory process of the document retrieval, the document retrieval control program  116  reads the location information file  108  and the weight file  109  which are stored in the magnetic disk  106 , into the work area  128  reserved in the main memory  110 . In the document retrieval process, as shown in a PAD of FIG. 6, at Step  700 , the query parsing program  117  is activated to parse the query expression entered from the keyboard  101 . If it is judged at Step  701  that a sentence or document is designated as the entered query expression, then at Step  702 , the predetermined character strings are extracted from the sentence or document designated as the query expression, by using the query term extraction program  118 . As this character string, words are used for a language such as English language in which each word is written in a delimiting manner, and for other languages, character strings obtained by dividing the different character types, consecutive n characters (“n” is a predetermined integer of 1 or larger), words derived with reference to a dictionary and the like, and so forth are used, as well as the character string extracted by the document registration process by the document registration program  112 . If it is judged at Step  701  that a sequence of words is designated as the entered query expression, then at Step  703 , this sequence of words is extracted from the entered query expression. Next, at Step  704 , the location information and weight acquisition program  119  is activated to acquire the location information and the weight of each character string (or word) extracted from the query expression, from the location information file  108  and weight file  109  read in the work area  128 . The acquired character string and weight are stored in the work area  128  by activating the weight storage program  120  at Step  705 . 
     Next, at Step  706 , the similarity calculation program  121  is activated to calculate the similarity between the query expression and the text data by using the location information and the weight of each character string acquired at Step  704  and by using the predetermined calculation method. Although this calculation method changes with the retrieval method to be used, the simple vector operation used in Related Art 1 may be used for this calculation. Lastly, at Step  707 , the search result list display program  122  is activated to arrange the text data in the descending order of the similarity calculated at Step  706 , and to display it as the search result list. Thereafter, the document retrieval process is terminated. 
     In displaying the document, the system control program  111  activates the document display control program  123  in response to the document display command from the keyboard  101 . The document display control program  123  controls a series of document display processes by using the query term arrangement program  124 , the string-for-changing-display-format extraction program  125 , the display format change program  126 , and the document display program  127 . The process contents of the document display process are illustrated in a PAD of FIG.  7 . As shown in FIG. 7, at Step  800 , the query term arrangement program  124  is activated to arrange the character strings and their weights, which are stored in the work area  128  by the document retrieval process of the document retrieval control program  116 , in the descending order of the weight. Next, at Step  801 , the string-for-changing-display-format extraction program  125  is activated to extract the upper m (“m” is a predetermined integer of 1 or larger) character strings of the character strings arranged at Step  800 . The value of “m” may be automatically set to a proper value by the system itself, or may be set by the user beforehand. Alternatively, the user may set and adjust the value to the proper value interactively each time the document is displayed. Next, at Step  802 , the display format change program  126  is activated to change the display format of portions which contain the character strings extracted at Step  801  in the document designated to be displayed (hereinafter called “a selected document to display”). The method of changing the display format is same as Related Art 2. Lastly, at Step  803 , the document display program  127  is activated to display the selected document to display in the display format changed at Step  802 . Thereafter, the document display process is terminated. 
     The summary of the process contents in the embodiment has been given above. 
     Next, a specific example of the process contents in the embodiment will be described in detail. 
     The contents of the document registration process by the document registration control program  112  are illustrated in FIG.  5 . The contents of this process will be described specifically. At Step  600 , the text registration program  113  is activated to read the text data of the document to be registered, from the floppy disk  105  which is inserted into the FDD  104 , and to store the read text data as the text  107  in the magnetic disk  106 . Next, at Step  601 , the location information file generation program  114  is activated to extract the predetermined character strings from the text data read at Step  600 . As the character strings to be extracted, words are used for a language such as English language in which each word is written in a delimiting manner, and for other languages, character strings obtained by dividing the different character types, consecutive n characters (“n” is a predetermined integer of 1 or larger), words derived with reference to a dictionary and the like, and so forth are used. Furthermore, the location information of each extracted character string is extracted and stored in the magnetic disk  106  as the location information file  108 . Although the location information changes with the retrieval method to be used, the location information necessary for the retrieval includes the serial number of the document which contains each character string, the location position thereof, the number of locations thereof and the like. In Related Art 1, the serial number of the document which contains each character string necessary for forming the vector of the document, and the number of locations thereof are used. An example of the process of generating the location information file  108  is illustrated in FIG.  8 . The location information file  108  shown in FIG. 8 stores the serial number of the document containing the word extracted by referring to the dictionary or the like, and the number of locations of each word contained in the document. For example, character string “W-Cup” has “2” locations (i.e., appears twice) in the document having the document serial number “1”. The document serial number “1” and the location number “2” are therefore stored in the location information file. Lastly, at Step  602 , the weight file generation program  115  is activated to calculate the weight of each character string extracted at Step  601  by using the predetermined calculation method, and to store it in the magnetic disk  106  as the weight file  109 . Although the calculation method changes with the retrieval method to be used, this weight is calculated by using the location frequency of each character string, the location deviation of each character string in the document database, or the like. The weight calculated from the location deviation is generally the IDF (Inverse Document Frequency) described in “Ranking Algorithms” by Donna Harman, Information Retrieval, pp. 363-392). The IDF is the weight proposed based on the concept that there is the high possibility that a character string contained in many documents is the stop word having the low importance degree. The weight file  109  shown in FIG. 8 stores the weights calculated by using the location deviation. Character string “football” which is contained only in the specific document has the high weight, and character string “next month” contained in many documents like the stop word has the small weight. Thereafter, the document registration process is terminated. 
     The contents of the document retrieval process by the document retrieval control program  116  are illustrated in FIG.  6 . The contents of this process will be described specifically. At Step  700 , the query parsing program  117  is activated to parse the query expression entered from the keyboard  101 . If it is judged at Step  701  that a sentence or document is designated as the entered query expression, then at Step  702 , the predetermined character strings are extracted from the sentence or document designated as the query expression, by using the query term extraction program  118 . As this character string, words are used for a language such as English language in which each word is written in a delimiting manner, and for other languages, character strings obtained by dividing the different character types, consecutive n characters (“n” is a predetermined integer of 1 or larger), words derived with reference to a dictionary and the like, and so forth are used as well as the character strings extracted by the document registration process by the document registration program  112 . An example is shown in FIG. 3 when query expression “Football match stadiums for W-Cup will be determined next month, selection right attributed to Association. The organizing arrangement committee for the 2002 football world cup under the joint auspices of Japan and Korea opened on 29-th a governor/mayor meeting by calling special directors from fifteen local self-governing bodies which are candidates for organizing stadiums. For the number of stadiums in Japan, Federation Internationale de Football Association (FIFA) . . . ” is designated. In the example shown in FIG. 3, nouns, katakana characters and gerunds are extracted by referring to the dictionary and the like as the character strings used for the retrieval. As a result, “football, W-Cup, match, stadium, next, month, determined, selection, right, Association, Japan; Korea, joint, auspices, world, cup, organizing, arrangement, committee, place, candidates, . . . ” is extracted from the query expression. If it is judged at Step  701  that a sequence of words is designated as the entered query expression, then at Step  703 , this word sequence is extracted from the query expression. If a logical condition of words, e.g., “searching a document containing both character strings of “football” and “world cup”” such as ““football” [AND] “world cup”” is designated as the query expression, then “foot ball” and “world cup” are extracted as the sequence of words. Next, at Step  704 , the location information and weight acquisition program  119  is activated to acquire the location information and the weight of each character string (or word) extracted from the query expression, from the location information file  108  and the weight file  109  read in the work area  128 . For example, when the location information and the weight of character string “football” are acquired from the location information file  108  and the weight file  109  shown in FIG. 8, the location information of “3” locations for the document and the weight of “80” are obtained for the document serial number “1”. This character string and this weight, e.g., the weight “80” of “football” are stored in the work area  128  by activating the weight storage program  120  at Step  705 . Next, at Step  706 , the similarity calculation program  121  is activated to calculate the similarity between the query expression and the text data by using the location information and the weight of each character string acquired at Step  704 , and by using the predetermined calculation method. Although this calculation method changes with the retrieval method to be used, the simple vector operation used by Related Art 1 may be used for this calculation. Lastly, at Step  707 , the search result list display program  122  is activated to arrange the text data in the descending order of the similarity calculated at Step  706  and to display the search result list such as shown in FIG.  3 . 
     It can therefore be understood that the document having the document serial number “123” has the highest similarity to the contents of the designated query expression. Thereafter, the document retrieval process is terminated. 
     The contents of the document display process by the document display control program  123  are illustrated in FIG.  7 . The contents of this process will be described specifically. At Step  800 , the query term arrangement program  124  is activated to arrange the character strings and their weights, which are stored in the work area  128  by the document retrieval process of the document retrieval control program  116 , in the descending order of the weight. The contents of this arrangement process are illustrated in FIG.  4 . With this arrangement process, the character strings are arranged in the descending order of the weight. For example, “W-Cup” and “football” having the weight “80”, “world cup” having the weight “70”, “FIFA” having the weight “60”, “place” having the weight 50, . . . are arranged. Next, at Step  801 , the string-for-changing-display-format extraction program  125  is activated to extract the upper m (“m” is a predetermined integer of 1 or larger) character strings of the character strings arranged at Step  800 . The value of “m” may be automatically set to the proper value in the system itself, or be set by the user beforehand. Alternatively, the user may set and adjust the value to the proper value interactively each time the document is displayed. In the example shown in FIG. 4, the upper four character strings are extracted, i.e., m=4. As a result, “W-Cup”, “football” “world cup” and “FIFA” are extracted as the character strings whose display format is changed. Next, at Step  802 , the display format change program  126  is activated to change the display format of the portions, which contain the character strings extracted at Step  801 , in the document designated to be displayed (hereinafter called “a selected document to display”). The method of changing the display format is same as Related Art 2. Lastly, at Step  803 , the document display program  127  is activated to display the selected document to display in the display format changed at Step  802 . The document display example is shown in FIG. 4 with the documents having the document serial numbers “123” and “003”. In this example shown in FIG. 4, the display format changed character strings “W-Cup”, “football”, “world cup” and “FIFA” are displayed with the larger size bold roman fonts. Thereafter, the document display process is terminated. 
     As described above, in this embodiment, the important character strings are selected from the character strings used in the retrieval to change only the display format of the portions which contain the important character strings. 
     If the document or the long sentence is designated as the query expression and the retrieved documents are displayed by changing the display format of all character strings used in the retrieval, a number of character strings are changed as shown in FIG.  3 . Therefore, there is the problem that the important character strings become difficult to be found. With the method of this embodiment, however, the display format can be changed only for the portions with the high importance. 
     Thereby, the user can confirm the important portions of the document at once, and can judge quickly whether or not the document is the desired document. The quality of the user interface for browsing the retrieved documents can therefore be improved. 
     In this embodiment, the display format of the character string having the high weight is changed. Not only the display format is changed, but also the information of the character string having the high weight such as a list of location frequencies may be displayed. It is possible to realize this by the similar processes to those described above. 
     Although the display format of the character strings is changed to the same display format in this embodiment, it is possible by the same processes described above to change their display formats in accordance with the weights of the character strings. For example, the character string having the highest weight is displayed with the large size and with the loud color. The character string having the next highest weight is displayed only with the large size. In this manner, it is possible to confirm the importance degree of each character string. 
     Next, a second embodiment of the invention will be described. 
     In this embodiment, when the character strings for the retrieval which are extracted from the sentence or document designated as the query expression are edited, the user selects the character strings whose display formats are changed in displaying the document (this display format change is hereinafter called “highlighting”). In this embodiment, only the character strings which the user considers to be important are highlighted when the document is displayed. Therefore, the user can confirm the important portions in the document at once, and can quickly judge whether or not the document is the desired document. 
     The second embodiment has fundamentally the same structure as that of the first embodiment (FIG.  1 ). However, the configurations of the document retrieval control program  116  and the document display control program  123  are different. FIG. 9 shows the structure of programs under the control of a document retrieval control program  116   a . FIG. 10 shows the structure of programs under the control of a document display control program  123   a . In response to the document retrieval command from the keyboard  101 , the system control program  111  activates the document retrieval control program  116   a . The document retrieval control program  116   a  controls the query parsing program  117 , a query term editing program  900 , a string-for-changing-display-format selection program  901 , the location information and weight acquisition program  119 , the similarity calculation program  121 , and the search result list display program  122 . In response to the document display command from the keyboard  101 , the system control program  111  activates the document display control program  123   a . The document display control program  123   a  controls a string-for-changing-display-format acquisition program  1000 , the display format change program  126  and the document display program  127 . 
     The contents of the document retrieval process by the document retrieval control program  116   a  and the contents of the document display process by the document display control program  123   a  which are different from the first embodiment will be described. 
     In retrieving the document, in response to the document retrieval command from the keyboard  101 , the system control program  111  activates the document retrieval control program  116   a . The document retrieval control program  116   a  controls a series of document retrieval processes by using the query parsing program  117 , the query term editing program  900 , the string-for-changing-display-format the selection program  901 , the location information and weight acquisition program  119 , the similarity calculation program  121  and the search result list display program  122 . For a preparatory process, the document retrieval control program  116   s  reads the location information file  108  and the weight file  109 , which are stored in the magnetic disk  106 , into the work area  128  reserved in the main memory  110 . In the document retrieval process, as shown in a PAD of FIG. 11, at Step  1100 , the query parsing program  117  is activated to parse the query expression entered from the keyboard  101 . If it is judged at Step  1101  that a sentence or document is designated as the entered query expression, then at Step  1102 , the predetermined character strings are extracted from the sentence or document designated as the query expression, by using the query term extraction program  118 . If it is judged at Step  1101  that a sequence of words is designated as the entered query expression, then at Step  1103 , this word sequence is extracted from the entered query expression. The above process contents are similar to those of the first embodiment. Next, at Step  1104 , the query term editing program  900  is activated to display the character strings extracted from the query expression. The user then adds or deletes the character strings used for the retrieval to the displayed character strings. At Step  1105 , the string-for-changing-display-format selection program  901  is activated to make the user select the character strings whose display format is changed when the document is displayed, from the character strings edited at Step  1104 . The contents of the selected character strings are stored in the work area  128 . Next, at Step  1106 , the location information and weight acquisition program  119  is activated to acquire the location information and the weight of each edited character string, from the location information file  108  and the weight file  109  which are read in the work area  128 . Next, at Step  1107 , the similarity calculation program  121  is activated to calculate the similarity between the query expression and the text data by using the location information and the weight of each character string acquired at Step  1106 , and by using the predetermined calculation method. Lastly, at Step  1108 , the search result list display program  122  is activated to arrange the text data in the descending order of the similarity calculated at Step  1107  and to display it as a search result list. The process contents from Step  1106  to Step  1108  are similar to the first embodiment. Thereafter, the document retrieval process is terminated. 
     In displaying the document, in response to the document display command from the keyboard  101 , the system control program  111  activates the document display control program  123   a . The document display control program  123   a  controls a series of document display processes by using the string-for-changing-display-format acquisition program  1000 , the display format change program  126 , and the document display program  127 . The contents of the document display process are illustrated in a PAD of FIG.  12 . As shown in FIG. 12, at Step  1200 , the document display control program  123   a  activates the string-for-changing-display-format acquisition program  1000  to acquire each character string to be changed its display format, which is stored by the document retrieval process by the document retrieval control program  116   a , from the work area  128 . Next, at Step  1201 , the display format change program  126  is activated to change the display format of portions which contain the character strings extracted at Step  1200  in the document designated to be displayed (hereinafter called “a selected document to display”). The method of changing the display format is same as Related Art 2. Lastly, at Step  1202 , the document display program  127  is activated to display the selected document to display in the display format changed at Step  1201 . Thereafter, the document display process is terminated. 
     The summary of the process contents of this embodiment has been given above. 
     Next, a specific example of the contents of the document retrieval process by the document retrieval control program  116   a  shown in FIG.  11  and the document display process by the document display control program  123   a  shown in FIG. 12 will be described in detail. 
     The contents of the document retrieval process by the document retrieval control program  116   a  are illustrated in FIG.  11 . The contents of this process will be described specifically. At Step  1100 , the query parsing program  117  is activated to parse the query expression entered from the keyboard  101 . If it is judged at Step  1101  that a sentence or document is designated as the entered query expression, then at Step  1102 , the predetermined character strings are extracted from the sentence or document designated as the query expression, by using the query term extraction program  118 . If it is judged at Step  1101  that a sequence of words is designated as the entered query expression, then at Step  1103 , this word sequence is extracted from the entered query expression. These process contents are similar to the first embodiment. Next, at Step  1104 , the query term editing program  900  is activated to display the character strings extracted from the query expression (hereinafter called “query terms”). The user then adds or deletes the character string used for the retrieval to the displayed character strings. At Step  1105 , the string-for-changing-display-format selection program  901  is activated to make the user select the character strings whose display formats are changed when the document is displayed (hereinafter called “a selected string to highlight”), from those character strings edited at Step  1104 . The contents of the selected character strings are stored in the work area  128 . An example of the display screen in the query term editing and the selected-string-to-highlight selection at Steps  1104  and  1105  is illustrated in FIG.  13 . In this screen shown in FIG. 13, a list of query terms is displayed. The user selects the character strings used for the retrieval, and turns on a check box displayed as “for retrieval”. The character string with the turned-on check box is used for the retrieval, and other character strings are excluded from ones to be retrieved. If a character string different from those displayed query terms is to be used for the retrieval, the user depresses an “add a character string” button, and enters a character string addition command. Thereby, the user adds a character string desired by the user. The selection of the selected string to highlight is realized by turning on a check box displayed as “for display”. The character string having the turned-on check box is displayed in the changed display format when the document is displayed. The display format includes color, size, font, style (bold or roman) and the like. After all settings are completed, the user depresses an “execute retrieval” button, and enters a retrieval execution command. In response to this, each character string with the turned-on check box is stored in the work area  128  to execute Steps  1106  to  1108 . At Step  1106 , the location information and weight acquisition program  119  is activated to acquire the location information and the weight of each edited character string, i.e., the character string with the turned-on check box for retrieval shown in FIG. 13, from the location information file  108  and the weight file  109  which are read in the work area  128 . Next, at Step  1107 , the similarity calculation program  121  is activated to calculate the similarity between the query expression and the text data by using the location information and the weight of each character string acquired at Step  1106 , and by using the predetermined calculation method. Lastly, at Step  1108 , the search result list display program  122  is activated to arrange the text data in the descending order of the similarity calculated at Step  1107 , and display it as the search result list. The process contents at Steps  1106  to  1108  are similar to the first embodiment. Thereafter, the document retrieval process is terminated. 
     The contents of the document display process by the document display control program  123   a  are illustrated in FIG.  12 . The contents of this process will be described specifically. At Step  1200 , the string-for-changing-display-format acquisition program  1000  is activated to acquire from the work area  128  each character string to be changed its display format, which is stored by the document retrieval process of the document retrieval control program  116   a . In the example of the display screen of the query term editing and the selected-string-to-highlight selection shown in FIG. 13, the user turns on the check boxes for display of “foot ball”, “W-Cup”, “stadium” and “world cup”, so that these character strings are acquired from the work area  128 . Next, at Step  1201 , the display format change program  126  is activated to change the display format of the portions which contain the character strings acquired at Step  1200  in the document designated to be displayed (hereinafter called “a selected document to display”). The method of changing the display format is same as Related Art 2. Lastly, at Step  1202 , the document display program  127  is activated to display the selected document to display in the display format changed at Step  1201 . A document display example is shown in FIG. 14 with the documents having the document serial numbers “123” and “003”. In this example shown in FIG. 14, the portions which contain “football”, “W-Cup”, “stadium” and “world cup” are displayed with the larger size bold roman fonts. Thereafter, the document display process is terminated. 
     As described above, in this embodiment, when the character strings for the retrieval extracted from the sentence or document designated as the query expression are edited, the user selects each character string whose display format is changed in displaying the document, and the display format of only each document portion which contains the selected character string is changed. If a document or a long sentence is designated as the query expression and the retrieved documents are displayed by changing the display format of all character strings for the retrieval, a number of portions are changed as shown in FIG.  3 . Therefore, there is a problem that the important portions become difficult to be found. With the embodiment method, however, it is possible to change the display format only for the important character strings selected by the user. 
     Thereby, the user can confirm the important portion in the document at once, and can judge quickly whether or not the document is the desired document. The quality of the user interface for browsing the retrieved documents can therefore be improved. 
     In this embodiment, the display format of the character string selected by the user is changed. Not only the display format is changed, but also the information of the character strings selected by the user such as a list of location frequencies may be displayed. This can be realized by the similar processes to those described above. 
     In this embodiment, the user selects the character string whose display format is changed. The display format of the character string added by the user when the query term is edited can also be changed. This an be realized by the similar processes to those described above. The added character string is considered to be important by the user, so that the important portions in the document can be confirmed at once by changing the display format of this character string in displaying the document. 
     If the designated query expression is the search result document selected from the previous search result list, the display format of the character string in the query expression used for retrieving the document is changed. This can be realized by the similar processes to those described above. For example, if the retrieval is executed by designating a word and the document retrieval is further executed by using the obtained document, the first designated word is the important work. Therefore, the display format of this word is changed when the document is displayed. In this manner, the important portion of the document can be confirmed at once. 
     Next, a third embodiment of the invention will be described. 
     In this embodiment, when a document is displayed, the display format of a character string which greatly contributes to the score of the document is changed (this display format change is hereinafter called “highlighting”). With this method, the user can confirm at once which character strings are used for calculating the score of the document, so that the user can judge at high speed whether or not the document is the desired document. 
     This embodiment has fundamentally the same structure as that of the first embodiment (FIG.  1 ). However, the configuration of programs under the control of the document display control program  123  is different. FIG. 15 shows the structure of programs under the control of the document display control program  123   b . In response to a document display command from the keyboard  101 , the system control program  111  activates the document display control program  123   b . The document display control program  123   b  controls a location information acquisition program  1500 , a similarity-every-character-string calculation program  1501 , the string-for-changing-display-format extraction program  125 , the display format change program  126  and the document display program  127 . 
     The contents of the document display process by the document display control program  123   b  which is different from the first embodiment will be described. 
     In displaying the document, in response to the document display command from the keyboard  101 , the system control program  111  activates the document display control program  123   b . The document display control program  123   b  controls a series of document display processes by using the location information acquisition program  1500 , the similarity-every-character-string calculation program  1501 , the string-for-changing-display-format extraction program  125 , the display format change program  126  and the document display program  127 . The document display process contents are illustrated in a PAD of FIG.  16 . As shown in FIG. 16, at Step  1600 , the document display control program  123   b  activates the location information acquisition program  1500  to acquire each character string and its weight which are stored in the work area  128 , by the document retrieval process of the document retrieval control program  116 . The document designated to be displayed (hereinafter called “a selected document to display”) is scanned to acquire the location information of each acquired character string in this document. This location information is the same as that extracted by the document registration process of the document registration control program  112 . 
     In Related Art 1, the number of the locations of the character string necessary for generating the vector of the document is extracted (since only the selected document is processed, the document serial number is not necessary as the location information). Next, at Step  1601 , the similarity-every-character-string calculation program  1501  is activated to calculate the contribution factor of each character string to the similarity of the selected document to display, by using the location information and the weight which are obtained at step  1601 . The character strings are arranged in the descending order of this contribution factor. The similarity of each selected document to display is a value representative of the similarity to the contents of the query expression which is calculated by the document retrieval process of the document retrieval control program  116 . Generally, the similarity uses the total sum of the values of each character string calculated by using the location information and the weight and by using the predetermined calculation method. The ratio of the calculated value of each character string to the similarity can be used as the contribution factor. Next, at Step  1602 , the string-for-changing-display-format extraction program  125  is activated to extract the upper m (“m” is a predetermined integer of 1 or larger) character strings of the character strings arranged at Step  1601 . The value of “m” may be automatically set to the proper value by the system itself, or may be set by the user beforehand. Alternatively, the user may set and adjust the value to the proper value interactively each time the document is displayed. A threshold value of the contribution factor may be set to extract a character string having the contribution factor equal to or larger than the threshold value. Next, at Step  1603 , the display format change program  126  is activated to change the display format of portions which contain the character strings extracted at Step  1602  in the selected document to display. The method of changing the display format is same as Related Art 2. Lastly, at Step  1604 , the document display program  127  is activated to display the selected document to display in the display format changed at Step  1603 . Thereafter, the document display process is terminated. 
     The summary of the process contents of this embodiment has been given above. 
     Next, the contents of the document display process of the document display control program  123   b  shown in FIG. 16 will be described in detail using a specific example. At Step  1600 , the location information acquisition program  1500  is activated to acquire each character string and its weight which are stored in the work area  128 , by the document retrieval process of the document retrieval control program  116 . 
     The selected document to display is then scanned to acquire the location information of the acquired character string in the selected document to display. This location information is the same as that extracted by the document registration process of the document registration control program  112 . In Related Art 1, the number of the locations of the character string necessary for generating the vector of the document is extracted (since only the selected document to display is processed, the document serial number is not necessary as the location information). FIG. 17 illustrates an example processed by this process. The character strings used for the retrieval “W-Cup”, “football”, “stadium”, “next month”, “determined”, . . . and their weights are acquired from the work area  128 . By scanning the selected document to display, by using these character strings, the location information is acquired. In this example shown in FIG. 17, the number of the locations in the document is used as the location information. Next, at Step  1601 , the similarity-every-character-string calculation program  1501  is activated to calculate the contribution factor of each character string in the document to the similarity. The character strings are arranged in the descending order of the contribution factor. The similarity of the selected document to display is a value representative of the similarity to the contents of the query expression, which is calculated by the document retrieval process of the document retrieval control program  116 . Generally, the similarity uses the total sum of the values of each character string calculated by using the location information and the weight, and by using the predetermined calculation method. As a result, the ratio of the calculated value of each character string to the similarity can be used as the contribution factor. In the example shown in FIG. 17, the contribution factor of each character string to the similarity of the selected document to display is calculated from the weights of the character strings “W-Cup”, “football”, “stadium”, “next month”, “determined”, . . . , and the number of the locations of these character strings in the document. The character strings are arranged in the descending order of the contribution factor to obtain as shown in FIG. 17 the character strings “W-Cup”, “football”, “stadium”, “next month”, “determined”, . . . . Next, at Step  1602 , the string-for-changing-display-format extraction program  125  is activated to extract the upper m (“m” is a predetermined integer of 1 or larger) character strings of the character strings arranged at Step  1601 . The value of “m” may be automatically set to the proper value by the system itself, or may be set by the user beforehand. Alternatively, the user may set and adjust the value to the proper value interactively each time the document is displayed. The threshold value of the contribution factor may be set to extract the character string having the contribution factor equal to or larger than the threshold value. An example of this process is illustrated in FIG.  18 . In this example shown in FIG. 18, the upper three character strings (i.e., m=3) are extracted. Therefore, “football”, “W-Cup” and “stadium” are extracted as the character strings whose display format is changed. Next, at Step  1603 , the display format change program  126  is activated to change the display format of the portions which contain the character strings extracted at Step  1602  in the selected document to display. The method of changing the display format is same as Related Art 2. Lastly, at Step  1604 , the document display program  127  is activated to display the selected document to display whose display format is changed at Step  1603 . In the example of the document with the document serial number “123” shown in FIG. 18, the portions which contain the character strings whose display format is changed, i.e., “football”, “W-Cup” and “stadium” are displayed in the large size bold roman fonts. In FIG. 18, the document having the document serial number “003” is also displayed. If the document with the document serial number “003” is designated as the selected document to display, “football”, “arrangement” and “next month”, are extracted as the upper three character strings having the higher contribution factors to the similarity. The document is displayed by changing the display format of the portions which contain these character strings. Thereafter, the document display process is terminated. 
     As described above, in this embodiment, when the document is displayed, the character string which greatly contributes to the score of the document is extracted, and the display format of only the portion which contains the extracted character string is changed. If a document or a long sentence is designated as the query expression and the obtained documents are displayed by changing the display  10  format of all character strings used for the retrieval, a number of character strings are changed as shown in FIG.  3 . Therefore, there is a problem that the important portions become difficult to be found. With the embodiment method, however, the user can confirm at once which character strings are used for calculating the score of the selected document to display, so that the user can judge at high speed whether or not the document is the desired document. The quality of the user interface for browsing the retrieved document can therefore be improved. 
     In this embodiment, the display format of the character string which greatly contributes to the score of the document is changed. Not only the display format is changed, but also the information of the character strings which contributes to the calculated score such as a list of the contribution factors may be displayed. This can be realized by the similar processes to those described above. By removing the character string contributing to the score calculation of the document which is not the desired document, from the character strings for the retrieval, it becomes possible to reduce the score of this document and to increase the relative score of the desired document. It is therefore possible to obtain the desired document at high speed. 
     According to the above-described embodiments, even if a number of the character strings are used for the retrieval, the character strings which affect the factor representative of the similarity to the contents of the query expression are selected, and only the relevant information is displayed. Therefore, the user can confirm the important portions in the document at once, and can quickly judge whether or not the document is the desired document. Even if a document or a long sentence is designated as the query expression to display the obtained documents, the document retrieval system can be realized which has the document display interface easy to judge whether or not the document is the desired document. 
     As described above, according to the invention, the document retrieval method and system can be realized which is easy to judge whether or not the document is the desired document.