Record systems often consist of two types of storage, one "high speed" and easily accessed, the other "low speed." Given costs associated with maintaining and accessing records and fixed costs for altering the configuration of records in the two types of storage, results are obtained demonstrating rules for determining which records are to be placed in fast storage, and the size of this storage.
Under present technological constraints, it is often impossible for a computer information system to keep all current active records in "high speed" storage. Therefore, it becomes necessary to place records less often used in some type of "slower" storage. Examples of high versus low speed storages are: core memory or disk vs. tape, online computer storage vs. offline microfilm, or a centrally located record room vs. "cold" storage in a less accessible location. For the following discussion we will refer to the two types of storage as "online" and "offline," although no restriction to computer systems is implied. If we assume that demand for the records in each period is stochastic, then there are two decisions that need to be made relating to the maintaining of such an information system: which records to keep online, and how much space to make available for such records.
The only reference to this problem is the recent article by Rockart (I) who presents methods of estimating the probability that a record is accessed. He goes on to describe the use of these probabilities. It is felt, however, that the methods discussed below provide a more rational approach to some aspects of the actual handling of these records and the record system. This paper answers some aspects of these problems in the next two sections. The final section discusses implementing such a system and a possible extension of the results. The Appendix contains proofs of the results included in earlier sections. THEMODEL Assume there is a population of records U, of which a subset N, is online and the remainder M = LT -N, are stored offline. (In the following the symbols U, N, and Copyright