The field of operations management has been criticized for the inadequacy of its theory. We suggest that this criticism may be too harsh, and further, that many building blocks of theory are prevalent in the body of existing research. This paper has two goals. The first is to suggest that careful organization of our thinking can lead to useful, productive theories in operations management that demonstrate all the hallmarks of the familiar theories of natural science. We discuss the nature of scientific inquiry in general terms, and examine the implications for what should be expected from theory in operations management. Our second goal is to illustrate through examples how such theories and their related laws might be developed. Two theories are proposed: the Theory of Swift, Even Flow, and the Theory of Performance Frontiers. The Theory of Swift, Even Flow addresses the phenomenon of cross-factory productivity differences. The Theory of Performance Frontiers addresses the multiple dimensions of factory performance and seeks to unify prior statements regarding cumulative capabilities and trade-offs. Implications drawn from the theories are discussed and concluding remarks suggest the advantages of future theory development and test. q 0272-6963r98r$ -see front matter q 1998 Elsevier Science B.V. All rights reserved.Ž. PII: S 0 2 7 2 -6 9 6 3 9 8 0 0 0 2 8 -X

The records of superior performance of selected service firms over many years suggest that they may be more productive than others. This article uses the Theory of Swift, Even Flow to explain why that might be true. In the process, this article improves Schmenner's 1986 service process matrix. The redefinition of the axes of this matrix and of the resulting diagonal leads to enhanced understanding of productivity for service operations and helps to explain how some leading service companies have been able to sustain their competitive positions for decades.Subject Areas: Productivity, Service Operations and Theory.

Order picking, the assembly of a customer's order from items in storage, is an essential link in the supply chain and is the major cost component of warehousing. The critical issue is to simultaneously reduce the cost and increase the speed of the order picking activity. This study departs from the limited prior research that focused on either routing of workers or storage of warehoused items. The main objectives are to (1) evaluate various routing heuristics versus an optimal routine in a volume-based storage environment, (2) propose several methods of implementing volume-based storage, and (3) examine the interaction of the routing and storage policies under different operating conditions of pick list size and demand skewness. The experimental results show statistically significant differences in the mean route distance for the routing policies, storage policies, and their interactions. Further testing indicates that the choice of certain routing and storage policies in combination can result in increased picking efficiency.

Purpose -The purpose of this paper is to explain why the historical integration of manufacturing with service was quickly seen as advantageous in some circumstances, but not so in others. Design/methodology/approach -The paper reviews the history of manufacturing companies in the USA during the last half of the nineteenth century, categorizes them, and ties them to theory. Findings -The bundling of manufactured goods to downstream-available services was led by companies with new products but with no great manufacturing strengths, as a way to establish barriers to entry. Companies with significant manufacturing capabilities were not as quick or as complete in their integration of manufacturing and service. Originality/value -This paper shows that servitization has antecedents that go back 150 years.