An apparatus acquires learning-data, including feature-elements, to which a label is assigned. The apparatus generates a first-set of expanded feature-elements by expanding the feature-elements. With reference to a model where a confidence value is stored in association with each of a second-set of expanded feature-elements, the apparatus updates confidence values associated with expanded feature-elements common between the first- and second-sets of expanded feature-elements, based on the label. Upon occurrence of an error indicating that a score calculated from the updated confidence values is inconsistent with the label, the apparatus sets a feature-size indicating a maximum size of expanded feature-elements to be used to update the model, based on the number of occurrences of the error for the acquired learning-data, and updates the model by adding, out of expanded feature-elements generated according to the set feature-size, expanded feature-elements unmatched with the second-set of expanded feature-elements, to the model.
Please help me write a proper abstract based on the patent claims. CLAIM: 1. A non-transitory computer-readable recording medium having stored therein a program for causing a computer to execute a process comprising: acquiring learning data to which a label of positive type or negative type is assigned, the learning data including feature-elements each configured as a feature or sub-structural data; generating a first set of expanded feature-elements by expanding the feature-elements included in the acquired learning data so that each expanded feature-element is configured as data generated from a combination of one or more feature-elements; with reference to a model in which a confidence value indicating a degree of confidence for a feature-element is stored in association with each of a second set of expanded feature-elements, comparing each of the first set of expanded feature-elements with the second set of expanded feature-elements stored in the model, and updating first confidence values associated with expanded feature-elements that are common between the first set of expanded feature-elements and the second set of expanded feature-elements stored in the model, based on a type of label assigned to the learning data; and upon occurrence of a classification error indicating that a type of a score calculated from the updated first confidence values is inconsistent with a type of the label assigned to the acquired learning data, setting a feature size indicating a maximum size of expanded feature-elements to be used to update the model, based on an error count indicating a number of occurrences of the classification error for the acquired learning data, and updating the model by adding, out of expanded feature-elements generated according to the set feature size, expanded feature-elements unmatched with the second set of expanded feature-elements stored in the model, to the model. 2. The non-transitory computer-readable recording medium of claim 1, the process further comprising: upon occurrence of the classification error occurs, updating the first confidence values, based on a type of the label assigned to the acquired learning data. 3. The non-transitory computer-readable recording medium of claim 1, wherein the sub-structural data is data that is represented by any one of an array structure, a tree structure, and a graph structure. 4. The non-transitory computer-readable recording medium of claim 3, wherein the learning data is configured as text data, and the sub-structural data is configured as partial character strings each common to plural pieces of the text data. 5. A method comprising: acquiring learning data to which a label of positive type or negative type is assigned, the learning data including feature-elements each configured as a feature or sub-structural data; generating a first set of expanded feature-elements by expanding the feature-elements included in the acquired learning data so that each expanded feature-element is configured as data generated from a combination of one or more feature-elements; with reference to a model in which a confidence value indicating a degree of confidence for a feature-element is stored in association with each of a second set of expanded feature-elements, comparing each of the first set of expanded feature-elements with the second set of expanded feature-elements stored in the model, and updating first confidence values associated with expanded feature-elements that are common between the first set of expanded feature-elements and the second set of expanded feature-elements stored in the model, based on a type of label assigned to the learning data; and upon occurrence of a classification error indicating that a type of a score calculated from the updated first confidence values is inconsistent with a type of the label assigned to the acquired learning data, setting a feature size indicating a maximum size of expanded feature-elements to be used to update the model, based on an error count indicating a number of occurrences of the classification error for the acquired learning data, and updating the model by adding, out of expanded feature-elements generated according to the set feature size, expanded feature-elements unmatched with the second set of expanded feature-elements stored in the model, to the model. 6. An apparatus comprising: a memory; and a processor coupled to the memory and configured to: acquire learning data to which a label of positive type or negative type is assigned, the learning data including feature-elements each configured as a feature or sub-structural data; generate a first set of expanded feature-elements by expanding the feature-elements included in the acquired learning data so that each expanded feature-element is configured as data generated from a combination of one or more feature-elements; with reference to a model in which a confidence value indicating a degree of confidence for a feature element is stored in association with each of a second set of expanded feature-elements, compare each of the first set of expanded feature-elements with the second set of expanded feature-elements stored in the model, and update first confidence values associated with expanded feature-elements that are common between the first set of expanded feature-elements and the second set of expanded feature-elements stored in the model, based on a type of label assigned to the learning data; and upon occurrence of a classification error indicating that a type of a score calculated from the updated first confidence values is inconsistent with a type of the label assigned to the acquired learning data, set a feature size indicating a maximum size of expanded feature-elements to be used to update the model, based on an error count indicating a number of occurrences of the classification error for the acquired learning data, and update the model by adding, out of expanded feature-elements generated according to the set feature size, expanded feature-elements unmatched with the second set of expanded feature-elements stored in the model, to the model