Patent Application: US-61043090-A

Abstract:
a method for storing and searching documents also useful in disambiguating word senses and a method for generating a dictionary of context vectors . the dictionary of context vectors provides a context vector for each word stem in the dictionary . a context vector is a fixed length list of component values corresponding to a list of word - based features , the component values being an approximate measure of the conceptual relationship between the word stem and the word - based feature . documents are stored by combining the context vectors of the words remaining in the document after uninteresting words are removed . the summary vector obtained by adding all of the context vectors of the remaining words is normalized . the normalized summary vector is stored for each document . the data base of normalized summary vectors is searched using a query vector and identifying the document whose vector is closest to that query vector . the normalized summary vectors of each document can be stored using cluster trees according to a centroid consistent algorithm to accelerate the searching process . said searching process also gives an efficient way of finding nearest neighbor vectors in high - dimensional spaces .

Description:
the document storage and retrieval and word sense disambiguation methods of the present invention are based upon a representation scheme using context vectors . a context vector is a fixed length vector having a component value for each of a plurality of word - based features . for using the methods of the present invention , a set of features that are useful for discriminating words and documents in a particular language and domain are required . a set of sample features are provided in table 1 . it is presently recommended that for use in the present invention , context vectors of between 150 and 500 features be used . the number and meaning of the features will be the same for all of the context vectors in a dictionary of context vectors for use in the present invention . table 1______________________________________human man woman machine politicsart science play sex entertainmentwalk lie - down motion speak yellresearch fun sad exciting boringfriend family baby country hotcold hard soft sharp heavylight big small red blackwhite blue yellow animal mammalinsect plant tree flower bushfruit fragrant stink past presentfuture hight low wood plasticpaper metal building house factorywork early late day nightafternoon morning sunny cloudy rainsnow hot cold humid brightsmart dumb car truck bikewrite type cook eat spicy . . . ______________________________________ a system can be built once the specified features have been determined . each word or word stem in a dictionary of words needs to have a context vector defined for it . a context vector is made up of component values each indicative of the conceptual relationship between the word defined by the context vector and the specified features . for simplicity , the values can be restricted to + 2 , + 1 , 0 , - 1 , - 2 . a component is given a positive value if its feature is strongly associated with the word . 0 is used if the feature is not associated with the word . a negative value is used to indicate that the word contradicts the feature . as an example , using the features in table 1 , the vector for &# 34 ; astronomer &# 34 ; might begin ______________________________________ & lt ; + 2 + 1 + 1 - 1 - 10 + 2 0 0 00 0 + 1 + 1 + 1 + 2 + 1 - 1 + 1 - 1 . . . . . . . . . . . . . . . & gt ; ______________________________________ under such a representation , &# 34 ; car &# 34 ; and &# 34 ; automobile &# 34 ; are expected to be very similar , &# 34 ; car &# 34 ; and &# 34 ; driver &# 34 ; somewhat similar , and &# 34 ; car &# 34 ; and &# 34 ; hippopotamus &# 34 ; uncorrelated . this is the essence of the word - based meaning sensitivity of the current invention , and it extends to document and query representations as discussed below . it is noted that the interpretation of components of context vectors is exactly the same as the interpretation of weights in neural networks . it would be wasteful to create context vectors for all words . uninteresting words such as a , an , the , or , for , etc . are removed from the dictionary . furthermore , only the stem of each remaining word is kept . for example , &# 34 ; investments &# 34 ; becomes &# 34 ; invest &# 34 ;. context vectors are thus only defined for the word stems . it is contemplated that the dictionary of context vectors could be created by hand . although , it is expected that such a task would be time consuming , once the task is completed it need not be completed again . thus , the brute force method may be used in which for each word in the dictionary of context vectors , a component value is manually selected for each feature in the context vector for that word . this is repeated until the context vectors are finished for each of the words in the dictionary . by limiting the dictionary to word stems , much redundant effort can be avoided . another possibility is to automate much of the task of context vector creation , as described below . as an option , context vectors may be lengthened to include random features in addition to the word - based features . for a random feature , the component values for each context vector are generated at random . the use of random features in context vectors will assist in keyword recognition . the more random features that are used , the more sensitive the system is to locating an actual search word . the fewer random features that are used , the more meaning - sensitive the system is . for example , without random features , a search for &# 34 ; car &# 34 ; and a search for &# 34 ; automobile &# 34 ; would have very similar results . but using random features , the two words would have vectors that are distinguishable by the random features and the searches would thus be more sensitive to appearance of the words themselves . referring now to the drawings , fig1 illustrates the document storage and retrieval system of the present invention using the context vectors . the system is operated by a computer processing system . documents 12 are entered into the processing system in machine readable form . the document storage subsystem 14 converts the documents into summary vectors 16 based upon the context vectors of the words in the document . the summary vectors 16 are stored for use in response to search requests . the document storage subsystem can be enhanced by arranging the summary vectors in accordance with a cluster tree . user queries 18 are converted to a vector for use by the retrieval system 20 in identifying responsive documents from the data base . a user query 18 may also be augmented by submitting selected documents 22 which are reduced to a summary vector such that the summary vector is then used as the query vector by the retrieval subsystem to obtain other documents similar to the selected documents . referring now to the document storage subsystem of fig2 and the summary vector creation method 19 of fig3 a summary vector is generated for each document 12 . a summary vector is a fixed length vector having a length equal to the number of features . this is the same length as the context vectors . the same process is performed for each document 12 in determining its summary vector . a summary vector of the fixed length is initiated 24 with all 0 component values . summary vector creation may be done for a single document or in the case of a query based on a plurality of texts or words , the summary vector is representative of all of the texts or words used in the query . an index t is set 26 to zero and then incremented 28 to count through all of the texts or words used in a query . if several texts and / or words are being used to form a summary vector each of the texts and / or words may be given a different weight 30 . when a summary vector is being created for a single document the weighting step is irrelevant . the weight of the single document would be made equal to 1 so that this step has no effect on the processing . in order to eliminate uninteresting words such as common function words and in order to find word stems for the remaining words , the document is preprocessed 32 as shown in fig4 . any common uninteresting words such as a , an , the , or , for , etc . are removed from consideration 34 . the remaining words are reduced to their stems by stripping off suffixes 36 . for example , investments becomes invest . any well known algorithm for reducing words to their stems may be used . ( 1 ) it may be possible to enhance the accuracy of the searching techniques by using additional processing on the documents . for example , a parsing algorithm can be used to identify the subject , predicate and verb in each sentence . the subject and verb or the subject , verb and predicate can then be assigned 38 a greater weight than the other words in each sentence . another method is to give the first 100 ( or so ) words in a document extra weight . other methods of assigning weights 38 to words in a document may also be used . there are well known algorithms based on the frequency of use of a word in a document or in a series of documents which may be used so as to assign a different weight to each of the remaining words in the document . for example , ( 1 , p . 304 ) stem s in document d might be weighted by tf ( d , s ) is the number of appearances of stem s in document d ; df ( s ) is the number of documents in which stem s appears . the preprocessed document may then be converted 40 into vector form as shown in fig5 . a summary vector is initialized 42 by setting all component values to 0 . each of the words remaining in the preprocessed text is considered one at a time 44 . for each word , its associated context vector is located 46 one at a time in a dictionary of context vectors . the context vector for the word is multiplied 48 by the word &# 39 ; s weight if weights were assigned 38 during preprocessing . this multiplication step 48 when used , produces weighted context vectors . the context vector or weighted context vector , as the case by be , is added 50 to the summary vector being formed for the document . for each feature in the vectors , the component value from the context vector of the word is added to the component value for the summary vector being formed . this results in a new summary vector for use with the next word in the document . after the context vectors for all of the remaining words to be considered in the document have been added , a gross summary vector 52 for the document is obtained . returning now to fig3 if a summary vector is being determined for a plurality of documents in a query , the gross summary vector obtained from a summation process can be multiplied by a weight and added to the summary query vector being formed 54 . summary vector creation may then take place for the next document 56 being used to form the query . when all the documents being used in the formation of the summary vectors have been processed , the gross summary vector is completed . the gross summary vector from the summation process is normalized 58 . normalization is performed by dividing each component value in the vector by the absolute magnitude of the vector . the magnitude of the vector is determined by taking the square root of the square of all of the component values in the vector . this results in a normalized summary vector . by providing normalized summary vectors , each document is given an equal weighting in a data base in which they are stored . the normalized summary vector is output 60 for storage . thus , a data base is collected with a normalized summary vector for each document in the data base . searches can be quickly conducted through the use of the normalized summary vector data base rather than requiring the exhaustive search through the entire contents of all the documents . as shown in fig2 the storage of the normalized summary vectors can be arranged to further reduce searching time by creating cluster trees . cluster tree formation 62 is described in greater detail with respect to fig6 . an initial parent node at the top of the tree indexed as level 0 , node 1 , contains all of the normalized summary vectors in the data base . a series of child nodes each branching from the initial parent node is created at a next level of the cluster tree . a centroid consistent clustering algorithm is used to divide the summary vectors among the series of nodes . a group of clusters is centroid consistent if every member of every cluster belongs to the cluster in the group with the closest centroid . a centroid is determined by taking , for each feature , the average of the component values from all of the context vectors in the group . one popular centroid consistent clustering algorithm is convergent k - means clustering . ( 11 ) convergent k - means clustering can be performed as follows : 1 . begin with any initial partition that groups the vectors into k clusters . for example , take the first k summary vectors as single element clusters . assign each of the remaining summary vectors with the cluster nearest centroid . after each assignment , recompute the centroid for the cluster which gains a vector ; 2 . take each summary vector in sequence and compute its distance from the centroid of each of the k - clusters . if the vector is not currently in the cluster with the closest centroid switch the vector to that cluster and update the centroids of the clusters which gain or lose a summary vector ; 3 . repeat step 2 until convergence is achieved , that is until a pass through all of the summary vectors causes no new assignments . since convergence may be rather time consuming to achieve , the clustering algorithm can be simplified by limiting the number of iterations through the algorithm . after say , 99 iterations of the algorithm , the centroids can be frozen . then one more pass can be made through all of the summary vectors distributing the vectors to appropriate clusters , but without updating the centroids . while , using this approximation , the centroids will no longer be exact centroids , the approximate centroids will be sufficient for the use of the present invention . it is not necessary to the present invention that the centroids be precise but rather that the clusters be centroid consistent . the last pass through the summary vectors guarantees that the clusters are centroid consistent with the approximate centroids . from herein , &# 34 ; centroids &# 34 ; as used in this application shall mean approximate centroids . in other words , a centroid sufficient to establish centroid consistent clusters . each node is identified by its centroid for use in the searching process . in forming a next level of clusters , the nodes in the level above become parent nodes to a set of child nodes below . only the summary vectors assigned to a parent node are used in the clustering algorithm to form the child nodes which branch from that parent . this is repeated across the entire level of parent nodes and on subsequent levels so that fewer and fewer context vectors are assigned to the child nodes on each lower level . the nodes form a tree pattern in which each node branches from a node in the level above . thus , each summary vector is assigned to a node on each level of the cluster tree . each node has a centroid . the bottom - level node assignments for each summary vector and the centroids for each node are stored for use in the search and retrieval algorithms . on the bottom level of the tree , each node points to each normalized summary vector assigned to it . the nodes on the bottom level may be referred to as buckets . once a cluster tree has been set up , it is a simple matter to add a new document summary vector to the tree . the initial branches of the tree are examined for the closest centroid . the summary vector is assigned to the node with the closest centroid . then the branches from that node are examined for the closest child node centroid , and the process is continued until a bucket is reached . the new document is then assigned to the bucket with the closest centroid of those buckets branching from the node on the previous level to which the summary vector was assigned . the centroids themselves are not changed . this action preserves centroid consistency of the clusters . if a bucket gets too big , the summary vectors on the bucket can be divided into sub clusters on a subsequent level . having stored all of the documents as summary vectors in a data base , we now turn to the document retrieval system of fig7 . an inquiry can be made using a set of words or terms or by submitting one or more documents for which similar documents are sought or a mixture of documents and words . weights may be assigned for each of the terms or documents in a query where such terms or documents are considered to be more or less pertinent to the results sought in the search . in order to treat a term comprised of several words with the same weight as a single key word , the context vectors of the words comprising the term are added together and then normalized to produce a single normalized context vector for the term . the query vector is obtained by weighting and summing the summary vectors of the query words and texts as described above with respect to fig3 . it is not necessary to normalize 58 the query vector . if the summary vectors in the data base are stored without the benefit of cluster trees 66 , the query vector is compared with each summary vector in the data base in a brute force manner to identify the summary vector which is closest 68 to the query vector . the relative distance between a query vector and a summary vector can be determined by multiplying the query vector by a summary vector . multiplication is performed by multiplying the component values for each feature together and summing the results . the result obtained can be compared with the magnitudes of the product vectors obtained with each of the summary vectors . the product vector with the maximum magnitude identifies the closest summary vector to the query vector . alternatively , the relative distance between the summary vectors and a query vector can be determined by subtracting the query vector from each of the summary vectors . the magnitude of the difference vectors may then be used to identify the closest summary vector to the query vector . however , in this case it is the difference vector with the minimum magnitude which is the closest summary vector . by using the cluster tree storage mechanism of the present invention , the searching task can be greatly accelerated . searching through a cluster tree 70 for the closest summary vector to a query vector is described with respect to fig8 and 9 . the query vector is used in the search routine of fig9 to identify the summary vector that is closest to the query vector . the search is performed using a depth first tree walk . a branch is followed down the tree taking the node at each level having the centroid closest to the query vector . the search proceeds down the tree until a bottom level node ( bucket ) without children is reached 76 . each of the summary vectors in the bucket is compared with the query vector to identify the closest summary vector 78 . the closest summary vector v is remembered and updated if during the search a closer summary vector is identified . before a subsequent node in the depth first tree walk is checked for a closest vector , first it is determined whether the node can be completely pruned . a node is pruned if it is not possible for a closer normalized summary vector to be assigned to the node than the closest normalized summary vector found so far without violating centroid consistency . suppose we are examining a node with centroid c &# 39 ; for pruning . if c is the centroid of any sibling node then if it is true that any vector closer to the query vector q than v ( closest vector found so far ) must be closer to c than c &# 39 ;, then we may prune the node with centroid c &# 39 ; as well as any nodes branching therefrom . this may be computed by comparing 82 the distance between c and c &# 39 ; with twice the sum of the distance between c and q and the distance between q and v . if the distance between c and c &# 39 ; is greater , then the node with centroid c &# 39 ; ( and descendents ) may be pruned . if not , the formula is repeated for the remaining sibling nodes since any one of them may permit pruning to proceed . if none of the sibling nodes achieve pruning of the node , then the search continues through the node with centroid c &# 39 ; and down into the subsequent level if there is one . by using the pruning formula 82 , a node can be pruned when any vector closer to the query vector than the closest vector v must be closer to the centroid c than to the centroid c &# 39 ;. therefore , that vector could not be assigned to node c or else it would violate centroid consistency . if this is a bottom node , then all of the summary vectors on the node must be checked 78 to determine whether any are closer than the closest vector found so far . if a closer summary vector is found , it will then become the closest summary vector 80 being remembered . thus , bottom nodes are thoroughly searched if not pruned . the search continues in a depth first tree walk pruning off entire branches when possible . these searches continue through the tree until all branches have either been checked or pruned . after the entire tree has been searched , the closest summary vector has been identified . the document associated with the summary vector can be retrieved . the pruning formula given above provides for rough pruning of the tree . greater pruning can be accomplished if more work is put into the pruning algorithm . when the simple pruning algorithm fails it may be desirable to use linear programming to attempt to prune the path . this would require additional computational time but it may be worthwhile for pruning a high level branch . for a linear programming approach , we seek to find out whether the following set of constraints has a feasible solution . suppose we are considering node n for pruning and v is the closest vector found so far . we check whether any vector v * can exist that satisfies : 1 . for each node n 1 in the tree path from the initial parent node to n , it must be that v * is closer to the centroid for n 1 than to the centroid for any other sibling node of n 1 ; and 2 . the distance between v q and v * is less than the distance between v q and v . these constraints can be formulated as a linear programming problem by one skilled in that art . if such problem is found to be infeasible ( i . e ., admit no solution ) then node n and descendents may be pruned . as shown in fig8 after the closest summary vector is found , it may be removed from consideration and the search repeated to find the next closest summary vector . this process may be repeated for as many summary vectors as are required . referring now to fig1 , the present invention is shown for use in achieving word sense disambiguation . the text surrounding an ambiguous word is input into 90 the processing system . a summary vector is then created 92 for the text surrounding the ambiguous word . summary vector creation was described with reference to fig3 . weights may be assigned to each of the words in the series of words . one weighting mechanism would be to give the greatest weight to words which are closest to the ambiguous word in the text . uninteresting words are removed from the series and the remaining words except for the ambiguous word are located in the dictionary of context vectors . the context vector for each of the remaining words is multiplied by its weight so as to produce a weighted context vector for each of the remaining words . for each of the remaining words being considered in the text surrounding the ambiguous word , the weighted context vectors are summed together . the sum of all of the weighted context vectors is the summary vector for the series of words . the normalization step is not necessary for word sense disambiguations . the word being disambiguated is then considered 94 . the dictionary of context vectors contains a different context vector for each of the different meanings which could be applied to the ambiguous word . the plurality of context vectors associated with the ambiguous word are retrieved from the dictionary of context vectors . the summary vector obtained from the surrounding text is then compared 96 with each of the context vectors associated with the ambiguous word . the relative distances between the summary vector and each of the context vectors can be determined by multiplying the vectors together or from subtracting the vectors from each other . the context vector which is determined to be closest to the summary vector of the surrounding text is identified as the appropriate meaning for the ambiguous word . if there are more than two possible meanings for the word , these can be ordered 98 according to their relative closeness to the summary vector for the surrounding text . the appropriate meaning can be output for the processing system . the foundation for the workings for the present invention is the dictionary of context vectors . at least part of the data base needs to be entered by hand . for each of the features making up all the features of the context vector an integer should be entered according to how that feature correlates , suggests and is consistent with the word stem for which the context vector is being formed . for example , a scale of from - 5 to + 5 may be used . it may be further advantageous to normalize the context vectors in the dictionary so that the average squared weight is the same for each feature . alternatively , normalization may be performed for each word so that the average squared weight is the same for each word in the dictionary . the creation of context vectors will be a time consuming task but fortunately it only needs to be performed once . due to the subtleties of human experience , it is preferred that at least a core group of words have their context vectors entered by humans . an automated method for building a dictionary of context vectors can be achieved with the aid of a training corpus 102 , i . e ., an initial set of documents , as shown in fig1 . for each word stem , the number of documents which the word stem appears in are counted 104 . we let f w be the fraction of training corpus documents in which the word stem w appears . all of the word stems are then ordered 106 by their information content which is defined by the equation : it is seen from this equation that words appearing in half of the documents have the highest information content while those appearing in either all or none of the documents have the lowest content . a core group of word stems having the highest information content are taken from the top of the list . for example , the first 1 , 000 word stems having the highest information content may be selected . for this core group of word stems , context vectors are generated by hand 108 . temporarily a 0 vector is assigned to any other word stems remaining 110 . the word stem w which has temporarily been assigned a 0 vector having the highest information content is then taken . for this word stem , the context vectors of word stems that are close to w in the training corpus documents are weighted by their distance from w . for example , the 10 stems preceding and following each occurence the word stem may be used . the weighted context vectors are added up to produce a context vector for the word stem 112 . the context vector can then be normalized 114 . the resulting context vector becomes w &# 39 ; s permanent context vector . the next word stem w having the highest information content from those word stems which have only a temporary 0 vector is then selected and the process is repeated 116 . it is recommended that at least 1000 documents be used . once the dictionary of context vectors is completed , the invention may be used to its full benefit . for such dictionary building , multiple meanings of a word do not enter in ; all stems have only one context vector . of course , it should be understood that various changes and modifications to the preferred embodiments described above will be apparent to those skilled in the art . for example , numerous weighting schemes , parsing algorithms , clustering algorithms or methods for creating a context vector dictionary are possible within the scope of the present invention . these and other changes can be made without departing from the spirit and scope of the invention and without diminishing its attendant advantages . it is therefore intended that such changes and modifications be covered by the following claims . 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