Push and pull in manufacturing and distribution systems

Abstract The terms “push” and “pull” have been used to describe a wide variety of manufacturing and distribution environments. To some, the distinction refers to a specific attribute which can be identified by observing the mechanisms for controlling material flow on the factory floor. To others push and pull can be defined in terms of a specific policy for the management of inventories and production schedules. Finally there are skeptics who maintain that the push/pull dichotomy is a fiction created by academics or unscrupulous consultants to promote their latest theories or systems. In this paper, we review the various definitions of push and pull in operations literature. It is clear from this review that the attributes which distinguish pull systems from push systems are not well understood. The purpose of this paper is twofold. First, we clarify the meaning of push and pull in manufacturing and distribution systems. This clarification gives rise to a framework for push and pull that can serve as a classification scheme for material control systems. While we do not claim that our framework sufficiently classifies all material control systems, we demonstrate its application to a number of common systems. Thus, this framework can be used to analyze and compare management procedures for different classes of material control policies (such as MRP, kanban, base stock). This analysis is particularly useful to designers of material control systems, as well as those doing empirical research, who need to consider in depth various aspects of material control. Second, we intend to stimulate research efforts by the use of our framework. We demonstrate one such use of the framework by introducing a model of a particular integrated production-distribution system. Our model represents a simple, three-location, production-distribution system with multiple products. We develop a system that is predominantly pull, and several systems with varying degrees of push. In the particular example we discuss, our pull system provides lower costs and higher service levels than the push systems. In developing our framework, we analyze material flow decisions in terms of four related decisions that can be used to distinguish push systems from pull systems: 1) batch size, 2) timing of a production or shipment request, 3) setting dispatch or allocation rules, and 4) the presence of interference mechanisms for expediting or handling of emergency orders. The first two decisions are primarily concerned with individual products or parts. The second two decisions introduce the added complexity of multiple products, locations and customers. For each decision, we examine the authority for the decision and the information content used in making the decision. We shall see that it is not possible or useful to label a manufacturing system as being entirely push or entirely pull. We argue that push and pull are characteristics of the underlying decision-making process; and hence manufacturing control systems, which embody a collection of decisions, will contain elements of push or pull to varying degrees. Nevertheless, certain systems, containing such diverse elements often give the impression that they are predominantly one or the other. Our extended definitions provide a more complete picture that can be useful to researchers engaged in empirical work, as well as to managers concerned with system design. The paper is organized as follows: following an introduction and a review of various definitions of push and pull, we describe integrated production-distribution systems. In the next section we present our framework for defining push and pull systems. Throughout this section we refer to commonly used planning and control systems, such as MRP, kanban, DRP, fair shares, and OPT. We then introduce a model that can be used to compare different material control systems. Finally, we present a summary and conclusions.