4. The method of claim 1, in which, in the step of generating a writer-specific reference vector, the writer-specific reference vector for each category is generated by correcting the reference vector for the category using the writer-specific feature vector for the category.
4. The method of claim 1, in which, in the step of generating a writer-specific reference vector, the writer-specific reference vector for each category is generated by correcting the reference vector for the category using the writer-specific feature vector for the category. 5. The method of claim 4, in which the step of correcting the reference vector for the category includes steps of: multiplying the reference vector for the category by a first constant matrix to obtain a first product; multiplying the writer-specific feature vector for the category by a second constant matrix to obtain a second product; and adding the first product to the second product to obtain the writer-specific reference vector for the category.