like u201c and u201d and u201c of u201d and u201c the u201d are taken as descriptors of the documents at first glance the tree structure generated as output of this prior art system appears to present meaningful categories but closer inspection reveals an important weakness in the system a category such as u201c camera u201d is indeed a useful category in that it divides the collection of items about u201c lens u201d and u201c lenses u201d in a meaningful way patents about camera lenses are likely to have significant commonalities when compared to patents about other types of lenses however categories such as u201c system u201d u201c apparatus u201d u201c device u201d and u201c method u201d clearly give very little information about the type of lens or lens patent contained therein methods for grinding lenses methods for selling lenses and methods for using lenses are grouped together under a u201c method u201d category moreover it may be seen from the example that the subcategories identified within the major category u201c optical u201d are virtually identical to the subcategories outside the category u201c optical u201d this is probably an indication that the presence or absence of the word u201c optical u201d in the title of a lens patent in this collection is not necessarily indicative that the lenses under discussion are other than optical in their construction and use in other words many of the categories created by this prior art methodology in the given example have little predictive power with respect to the contents of the categories so described beyond the presence or absence.