Note from the Institute – the Journal reprinted this seminal article by Professors Hopp & Spearman which explains the fundamental concepts from the Lean Philosophy from the theoretical framework of operations science. It is well-known in academic circles but deserves to be more widely-known among practitioners.
The terms pull and lean production have become cornerstones of modern manufacturing practice. However, although they are widely used, they are less widely understood. In this paper, we argue that while the academic literature has steadily revealed the richness of the pull/lean concepts, the practitioner literature has progressively simplified these terms to the point that serious misunderstandings now exist. In hopes of reducing confusion, we offer general, but precise definitions of pull and lean. Specifically, we argue that pull is essentially a mechanism for limiting WIP, and lean is fundamentally about minimizing the cost of buffering variability.
In recent years, both of us have been finding increasing confusion about how pull is defined and decreasing willingness to accept our definition. The main controversy has usually stemmed from some participants (who have almost always received lean training prior to attending one of our classes) insisting pull means making products to order, as opposed to making them to stock or forecast. Ten years ago, our students equated pull with Kanban, a make-to-stock system, but not with make-to-order. Nevertheless, it has been a disturbing experience for us because we do not feel that pull is properly defined as either Kanban or make-to-order.
Have we been wrong all along? The sheer number of confrontations in our classes forced us to consider that possibility. To decide, we revisited fundamental issues we thought were long resolved, such as what constitutes a pull system? What makes pull work? What is lean, and how does it relate to pull? In addition, we carefully reviewed the practitioner and academic literatures related to pull. This paper is the result of our soul-searching process. In it, we: (1) Provide a history of pull, from its antecedents up through recent history when trends caused the distinction between push and pull to become confused. (2) Describe the essence of pull and how “strategic pull” differs from “tactical pull.” (3) Investigate the relationship of pull and push to the concepts of make-to-order and make-to-stock. Although we cite a significant amount of literature that is important to understanding pull and its history, this paper is not intended as a comprehensive review of the pull literature.
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