Metric: A Multi-Echelon Model

In 1966, while at the RAND Corporation, the author developed the Multi-Echelon Technique for Recoverable Item Control (METRIC) for the Air Force (Sherbrooke 1968). It extends the single-site model discussed in Chapter 2. First-indenture items (i.e. parts that are installed directly on aircraft) are usually repairable (or recoverable), and they tend to be expensive, with low demand at any particular base. Thus, there is little batching of repair at the base level or batching of resupply requests on depot for multiple units of a given item. The one-for-one repair at base or resupply from depot simplifies the mathematics of the base-depot joint optimization problem. It turns out that the inventory problem can be addressed with results from the queueing theory literature.

The METRIC theory is the basis for a large number of multi-echelon models used by the several military services. In Chapter 4 we show how to model the multi-indenture problem (where the repair of first-indenture line replaceable units (LRUs) can generate demands for lower indenture parts), and how this can be made more accurate by taking into account the dependencies between indentures using VARI-METRIC theory. Then in Chapter 5 the VARI-METRIC theory is applied to the combined multiechelon, multi-indenture problem. The mathematical details underlying METRIC are kept to a minimum in this chapter, because these are provided in Chapter 5, when we derive the more accurate VARI-METRIC theory.