Opportunity-based age replacement: Exponentially distributed times between opportunities

This article gives a full analysis of a component-replacement model in which preventive replacements are only possible at maintenance opportunities. These opportunities arise according to a Poisson process, independently of failures of the component. Conditions for the existence of a unique average optimal control limit policy are established and an equation characterizing the optimal policy and minimal average costs is derived. An important result is that the optimal policy can be described as a so-called one-opportunity-look-ahead policy. Such policies play an important role as heuristics in more general models. It is shown that there is a correspondence with the well-known age-replacement model, which can be considered as an extreme case of the model. Finally, some numerical results are given.     

On excess-, current-, and total-life distributions of phase-type renewal processes

Consider a renewal process whose interrenewal-time distribution is phase type with representation (α, T). We show that the (time-dependent) excess-life distribution is phase type with representation (α′, T), where α′ is an appropriately modified initial probability vector. Using this result, we derive the (time-dependent) distributions for the current life and the total life of the phase-type renewal process. They in turn enable us to obtain the equilibrium distributions for the three random variables. These results simplify the computation of the respective distribution functions and consequently enhance the potential use of renewal theory in stochastic modeling—particularly in inventory, queueing, and reliability applications. © 1992 John Wiley & Sons, Inc.     

Cycles and trends in the procurement system: Anticipating changes in acquisition legislation, regulation and policy

In conclusion, let me raise the question of whether the Law Panel can impact these cycles or reverse these trends. If they are regarded as historical imperatives, the answer is clearly No. But we have been urged to be bold and to try to devise a new, simplified statute that will take us back to a better day. We are going to do that, without regard to the question whether Congress will accept the solution.

There probably has never been a better time for the Panel's report than the present; the window of opportunity is open. But what the Panel recommends is going to be awfully hard to sell, even in a window-of-opportunity year. If the predictions come true about the defense budget's going down to the level of the period when Eisenhower remained a Major for 14 years, maybe Congress will be receptive to anything that adds a little bit of value to the defense dollar. With this in mind, the Law Panel is likely to be very bold in its attempt to streamline the procurement system and reduce oversight. Hope springs eternal . . . .     

Landmarks in defense literature

Achtung-Panzer! By Major General Heinz Guderian, Translated by Christopher Duffy with Introduction and Notes by Paul Harris. (Arms & Armour Press £16.99. ISBN 1-85409-138-7, 220 pages)     

More ado about economic order quantities (EOQ)

This paper is concerned with the determination of explicit expressions for economic order quantities and reorder levels, such that the cost of ordering and holding inventory is minimized for specific backorder constraints. Holding costs are applied either to inventory position or on-hand inventory, and the backorder constraint is considered in terms of the total number of backorders per year or the average number of backorders at any point in time. Through the substitution of a new probability density function in place of the normal p.d.f., explicit expressions are determined for the economic order quantities and the reorder points. The resulting economic order quantities are independent of all backorder constraints. It is also concluded that under certain conditions, the minimization of ordering costs and inventory holding costs (applied to inventory position), subject to a backorder constraint, is equivalent in terms of reorder levels to minimization of the safety level dollar investment subject to the same backorder constraint.     

Inventory systems with imperfect demand information

An inventory system is described in which demand information may be incorrectly transmitted from the field to the stocking point. The stocking point employs a forwarding policy which attempts to send out to the field a quantity which, in general, is some function of the observed demand. The optimal ordering rules for the general n-period problem and the steady state case are derived. In addition orderings of the actual reorder points as functions of the errors are presented, as well as some useful economic interpretations and numerical illustrations.