Efficient search of items in a store of information, such as books in a library, abstracts of articles, information in a computer data bank or the like, requires that the items most closely connected in content be close together, in order that a person looking for some particular information may concentrate his search on as small a part of the store as possible. The degree of connectedness between two items, I and J, can be expressed in terms of a correlation index N sub I J. Methods of estimating these indexes are discussed. Once the correlation indexes are known for each pair of items in the store, it is possible to assign a position X sub I for each item I, along a linear classification scale, such that the larger the N sub I J between items I and J the closer the two items are on the scale. Such a scale would indicate the optimal ordering of books on library shelves or of items in any other data collection. It also would provide means for assigning optimal subject descriptors for a computer-based information store. A procedure is described whereby the values of the X sub I's may be determined in terms of the N sub I J's. (Author)