JOB-SHOP VS. HYBRID FLOW-SHOP ROUTING IN A DUAL RESOURCE CONSTRAINED SYSTEM

This paper studies the impact of job routing pattern on the performance of dual resource constrained (DRC) systems. Three markedly different job routing patterns are simulated using a DRC system model that incorporates labor transfer times. The problem is viewed from two perspectives: one portion of the analysis focuses on the relative performances of five different dispatching rules as the routing pattern varies; the other examines the effect of the routing pattern on each rule's individual performance. One generalized conclusion of this study is that routing pattern has no significant impact on the relative effectiveness of various dispatching rules. Consequently, findings from pure job-shop research studies may be applied to situations where a mixture of job-shop and flow-shop routings is present.     

SOME COMMENTS ON “A SYSTEMS PERSPECTIVE ON MATERIALS REQUIREMENTS PLANNING”*

We believe there are some serious deficiencies in the Morecroft article on MRP problems which appeared in a recent issue of this journal. The article, at the most, shows that MRP, inappropriately imposed on an existing production system, can result in undesirable system performance. This does not represent a new or startling conclusion. Four problem areas in the article are discussed, and suggested alternative approaches are offered.     

“A JOINT ECONOMIC-LOT-SIZE MODEL FOR PURCHASER AND VENDOR”: A COMMENT*

Banerjee [1] developed a joint economic-lot-size model for the case where a vendor produces to order for a purchaser on a lot-for-lot basis under deterministic conditions. The assumption of lot-for-lot bases is restrictive in nature. In this note, a more general joint economic-lot-size model is suggested and it is shown to provide a lower or equal joint total relevant cost as compared to the model of Banerjee.     

COMMENTS ON “COST INCREASES DUE TO DEMAND UNCERTAINTY IN MRP LOT SIZING”: A VERIFICATION OF ORDERING PROBABILITIES

In a recent article in this journal, De Bodt and Van Wassenhove [1] presented analytic derivations related to lot-sizing behavior under uncertainty. Although their models appear to have been verified in the aggregate by simulation experiments, detailed justifications for several of the derivations are missing. The present paper looks at De Bodt and Van Wassenhove's analysis and provides verifications of (and corrections to) the ordering probabilities and order cycles used by the authors to estimate the cost effects of forecast errors in the particular operating environment studied. The probabilities simulated in this study also generate additional insight into the “system nervousness” caused by lot-sizing and forecast errors.     

PROCEDURES FOR DETERMINING RELATIVE FREQUENCIES OF PRODUCTION/ORDER IN MULTISTAGE ASSEMBLY SYSTEMS*

We show that the problems of determination lot sizes in a multistage assembly system for the case of instantaneous production and constant demand for the end item can be reduced to the problems of determining relative frequenceis of production/order for items at each production stage. We further show that such frequencies are independent of the demand levels. Optimal and near-optimal solution procedures for this reduced problem are provided. The near-optimal procedure successively treats each stage of production as a final production stage while simulatenously incorporating decisions made at lower stages into decisions made at higher stages. Experimental results show that the near-optimal procedure results in optimal solutions 75 percent of the time and performs considerably better than representative heuristics available in the literature. Further, its performance is relatively less susceptible to product/structural characteristics of the system.     

Scheduling with Sequencing Flexibility*

This study examines the effects of sequencing flexibility on the performance of rules used to schedule operations in manufacturing systems. The findings show that taking advantage of even low levels of sequencing flexibility in the set of operations required to do a job results in substantial improvement in the performance of scheduling rules with respect to mean flowtime. Differences in the mean flowtime measure for various rules also diminish significantly with increasing sequencing flexibility. Performance improvements additionally result for such due-date related performance measures as mean tardiness and the proportion of jobs tardy. At high levels of sequencing flexibility, some nonparametric scheduling rules outperform the shortest processing time rule in terms of the mean flowtime criterion. Rules based on job due dates also outperform rules based on operation milestones in terms of tardiness related criteria at high levels of sequencing flexibility. The implications of these findings for the design of manufacturing systems and product design are noted.     

A Note on “An Efficient Zero-One Formulation of the Multilevel Lot-Sizing Problem”

In a recent paper, McKnew, Saydam, and Coleman [3] presented a novel zero-one integer programming formulation of the multilevel dynamic, deterministic lot-sizing problem in assembly systems. They stated that “the relaxed linear programming solution to this formulation will always be integer’ [3, p. 280] since the constraint matrix is totally unimodular. In this note, we point out that the constraint matrix is not totally unimodular and therefore the authors’claim that a linear relaxation of the zero-one integer formulation always yields an integer solution is not true.     

A COMPARISON OF SEQUENCING RULES FOR A TWO-STATE HYBRID FLOW SHOP

A common manufacturing environment in many industries (such as the glass, steel, paper, costume jewelry, and textile industries) is a hybrid flow shop. This system has continuous-process machinery in the fist state of manufacturing and repetitive-batch equipment in the second. Little research has investigated this type system. Scheduling managers of hybrid flow shops tend either to use existing job-shop rules or to devise their own rules. These approaches often are less than adequate for efficient scheduling. In this paper we extend the rule presented by Narasimhan and Panwalker [4] to include a general class of hybrid flow shops. This extenstion, called the generalized cumulative minimum-deviation (GCMD) rule, is compared under various operation conditions to three other sequencing rules: shortest processing time, longest processing time, and minimum deviation. The operating conditions are determined by the number of machines at both stages. The results of 7200 simulation runs demonstrate that the GCMD rule is better than the other rules in minimizing each of five chosen criteria. Thus, the GCMD rule can help managers to schedule hybrid flow shops efficiently to achieve various corporate objectives.     

MODELS OF PARTS-ORIENTING SYSTEMS*

An analytical model to estimate, evaluate, and compare the behavior and performance of parts-orienting systems for automated manufacturing systems is developed. The model can be applied to parts-orienting systems for both traditional series-structured manufacturing systems and nontraditionally structured flexible manufacturing systems. Numerical examples are given for typical system configurations such as the pure series system, the pure parallel system, and the pure feedback system.     

THE IMPACT OF EXTREME OBSERVATIONS ON SIMPLE FORECASTING METHODS*

In general linear modeling, an alternative to the method of least squares (LS) is the least absolute deviations (LAD) procedure. Although LS is more widely used, the LAD approach yields better estimates in the presence of outliers. In this paper, we examine the performance of LAD estimators for the parameters of the first-order autoregressive model in the presence of outliers. A simulation study compared these estimates with those given by LS. The general conclusion is that LAD does not deal successfully with additive outliers. A simple procedure is proposed which allows exception reporting when outliers occur.     

A METHODOLOGY FOR LABOR SCHEDULING IN A SERVICE OPERATING SYSTEM

In the increasingly competitive services sector, utilization of the labor force can make the difference between profits or losses. Until recently, service operations managers had a limited set of tools, most of them computer-based, for scheduling labor. This paper offers a manual heuristic for labor scheduling that outperforms traditional algorithmic solution approaches. Specifically, this study examines the problem of scheduling employees in service delivery system subject to demand variability. The manual heuristic proposed asigns full-time empolyees to weekly work schedules with the objective of minimizing the total number of labor hours scheduled. The performance of the manual heuristic is compared to the classical algorithmic solution and to a lower bound for a variety of demand distributions and system operating conditions. The heuristic is shown to produce a smaller work force than the classical approach in 106 of the 108 demand-operating condition patterns examined.     

THE CLASSIC ECONOMIC PRODUCTION QUANTITY MODEL WITH SETUP COST AS A FUNCTION OF CAPITAL EXPENDITURE*

The formulation of the classic economic production quantity (EPQ) model is extended to include setup cost as a function of capital expense. Additional capital will buy reduced setup cost. Thus, the objective now is to balance holding, setup, and capital expenses. This new formulation is solved under conditions where setup cost varies exponentially and linearly as a function of capital expense. Decision rules are formulated to indicate under what conditions setup cost reduction reduces total cost. For the linear function, it is shown that once the decision to reduce setup cost is justified, the optimal choice is the minimum setup cost that is technologically feasible.     

DESIGNING HIGH-CONTACT SERVICE SYSTEMS: APPLICATION TO BRANCHES OF A SAVINGS AND LOAN*

The customer-contact model represents one of the few conceptually based approaches for designing service systems. This paper develops that concept further for high-contact systems, relative to efficiency and effectiveness notions. Four design features (contact technologies, contact training, data bases, and contact roles) are developed and a simple design methodology (built around the four design features) is presented. A demonstration of the design methodology is provided through its applications to branches of a savings and loan. Areas for additional theoretical and practical refinement of the model and the desirability of multidisciplinary research are discussed.     

A Reexamination of ‘Measuring the Congruence Between Market Requirements and Manufacturing: A Methodology and Illustration'*

This paper reexamines the idea of developing an index of manufacturing fit with market requirements. Bozarth and Berry's (1997) recent work provides a starting point. Several conceptual and methodological issues relating to the Bozarth and Berry approach are raised. It is argued that the indifference profile concept suggested by the authors bears reexamination.     

An Investigation of Scheduling Policies in a Dual-Constrained Manufacturing Cell*

In previous group scheduling studies, labor has essentially been ignored by assuming that enough labor is assigned to each machine in the cell. In reality, however, management usually does not have the resources to employ a laborer at each machine in the cell (i.e., machines need to share labor). Both labor scheduling and group scheduling heuristics need to be administered to manage the cell effectively. This study develops and examines scheduling procedures for a dual-constrained (i.e., machine and labor) manufacturing cell. Eleven decision rules are developed and tested under 16 different experimental conditions. The experimental factors used are interarrival time distribution, cell load, setup-to-run-time ratio, and transfer-to-run-time ratio. Results show that interarrival time distribution and cell load have the greatest impact on the performance of the cell. This suggests that effective production planning aimed at reducing job arrival variation and leveling the cell load can substantially improve cell performance. Among the experimental factors, setup and transfer-to-run-time ratio factors had the strongest influence on the rankings of the decision rules. These rankings were fairly robust across all experimental conditions and performance measures. The results also indicated that the inclusion of labor as a contraint in the cell had a significant impact on the performance of several group scheduling heuristics. Finally, it was shown that the best performing decision rules consider both transfer time and subfamily setup times.     

Application of the Net Present Value Criterion in Random and Flow Shop Scheduling*

While the majority of the literature on shop scheduling has emphasized time-based performance criteria such as mean flow time, lateness, and tardiness, the primary goal of management should be the maximization of shop profitability. In this research the net present value (NPV) criterion is introduced to measure shop profitability. This measure combines aspects of job flow time and inventory holding costs into a single measure. A simulation model of a job shop is used to examine the performance of a variety of time- and value-based scheduling rules. These rules are evaluated with respect to the NPV criterion in both random and flow shop environments. The results suggest that priority rules that utilize monetary information about jobs yield a higher NPV than many time-based rules in most situations, with little sacrifice in job tardiness. A well-researched time-based rule, critical ratio, also provides excellent performance when the shop is heavily loaded.     

Concepts,Theory, and Techniques THE LOT-SIZE REORDER-POINT MODEL WITH UPSTREAM-DOWNSTREAM UNCERTAINTY*

One assumption in the classical lot-size problem is certainty in the amount requisitioned. In many practical situations, however, the amount received may be a random variable. In this paper, we discuss a lot-size inventory problem in which the quantity received does not necessarily match the quantity requisitioned. We develop explicit and approximate solutions for the back-orders case by assuming that the standard deviation of the amount received is linearly related to the quantity requisitioned.     

SOCIAL LEARNING THEORY AND THE IMPLEMENTATION OF PRODUCTION INNOVATION*

In a study of the implementation of production innovation using a social learning theory focus, four organizations were monitored with a real-time study design. The following propositions were strongly supported: (1) During the initial implementation period (3–4 months), the stress felt by personnel in new user organizations was significantly higher (p < .01) than for organizations previously experienced with technology; (2) Firms experienced with the new technology obtain early success when compared with inexperienced firms (p < .01). Moderate support was found for the proposition that new users rely heavily on original equipment manufacturers for demonstration to acquire new skills, but gradually models within the user organization emerge as the dominant influence. The proposition that new users have significantly higher imitation scores than users experienced with the technology was only weakly supported. No substantive or significant changes in the attitudes of users toward the innovation were found during the first three to four months of the implementation period. These and other results suggest that longitudinal studies of this type ought to be carried out over periods in excess of 2 1/2 years. Implications for the management of change are presented.     

Partitioning Work Centers for Group Technology: Insights from an Analytical Model*

This paper develops analytical approximations based on an M/M/c queuing model, for the operating characteristics of job shop work centers. The model is used to analyze the effects of partitioning a work center and introducing setup reduction into the cell components. The results using this model are shown to be consistent with the results of prior simulation-based studies of group technology under certain parameter ranges. A simulation experiment was conducted to verify the effects with multi-item, non-Markovian assumptions, and the general effects predicted by the model were found to apply.