We present a shifting bottleneck heuristic for minimizing the total weighted tardiness in a job shop. The method decomposes the job shop into a number of single‐machine subproblems that are solved one after another. Each machine is scheduled according to the solution of its corresponding subproblem. The order in which the single machine subproblems are solved has a significant impact on the quality of the overall solution and on the time required to obtain this solution. We therefore test a number of different orders for solving the subproblems. Computational results on 66 instances with ten jobs and ten machines show that our heuristic yields solutions that are close to optimal, and it clearly outperforms a well‐known dispatching rule enhanced with backtracking mechanisms. © 1999 John Wiley & Sons, Inc. Naval Research Logistics 46: 1–17, 1999

We present and compare a number of branch and bound algorithms for minimizing the total weighted tardiness in job shops. There are basically two types of branching schemes. The first one inserts operations in a partial schedule, while the second one fixes arcs ' in the disjunctive graph formulation of the problem. The bounding schemes are based on the analysis of precedence constraints, and on the solution of nonpreemptive single machine subproblems that are subject to so-called delayed precedence constraints. We obtain optimal solutions for all the instances with ten jobs and ten machines that we consider, including three tardiness versions of a well-known 10 x 10 instance introduced by Muth and Thompson 111 in 1963.

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Abstract:We present a shifting bottleneck heuristic for minimizing the total weighted tardiness in a job shop. The method decomposes the job shop into a number of single-machine subproblems that are solved one after another. Each machine is scheduled according to the solution of its corresponding subproblem. The order in which the single machine subproblems are solved has a significant impact on the quality of the overall solution and on the time required to obtain this solution. We therefore test a number of different orders for solving the subproblems. Computational results on 66 instances with ten jobs and ten machines show that our heuristic yields solutions that are close to optimal, and it clearly outperforms a well-known dispatching rule enhanced with backtracking mechanisms.

Scientific literature reports scarce evidence of whether Internetbased procurement systems improve the efficiency of State purchases. We propose a methodology to estimate savings in: (i) the centralization of administrative tasks, and (ii) price differentials due to a larger number of contractors and suppliers bidding on contracts. We test our methods with ChileCompra, the Chilean e-procurement agency. During 2007, 885 Chilean State agencies used this system to purchase US$4.5 billion in products and services. Our preliminary results show price reductions of 2.65% and administrative cost savings of 0.28%-0.38% between 2006 and 2007.3) Reduce administrative cost by improving the procurement process; and ----------------------------

Heinrich's pyramid, which postulates that the severity distribution of occupational accidents is relatively constant, is one of the main tools for management of occupational safety. However, existing evidence suggests that decreases in minor accidents have not always been followed by similar decreases in serious and fatal accidents. To test the statistical validity of the pyramid, we propose a Bayesian two-part model: the first part estimates accident rates using a Poisson-gamma distribution, and the second part estimates the proportion of minor, serious and fatal accidents using a Multinomial-Dirichlet distribution. If this proportion does not change when the accident rate is reduced, then the statistical validity of the pyramid is confirmed, but if it changes, then its validity is refuted. Our data cover more than 50,000 companies observed over 28 months in Chile. Heinrich's pyramid is confirmed to be statistically invalid for different economic activity sectors and geographic regions, but the discrepancy is so small that, for practical purposes, the pyramid is valid. We thus conclude that the occurrence of minor accidents is a useful signal for assessing and forecasting the overall safety performance of a firm.

PurposeThe existing business paradigm in the sawmill industry is that improvements are achieved by enhancements at each individual sawmilling plant. The authors investigate a collaborative approach, and propose a model for optimizing production and inventory planning decisions within a system of plants.Design/methodology/approachThe methodology models the sawmill production process in terms of two transformation stages and two inventory stages. The authors then extend the model to account for supply chain decisions consisting of timber transfers among plants. They apply the model at 11 Chilean sawmills to evaluate whether they can benefit from operating in a more integrated manner.FindingsThe authors find an opportunity to increase profits by more than 15 percent through a higher utilization of the capacity and a better assignment of production orders.Research limitations/implicationsThe study neglects the possibility of exchanging raw material among plants. Also, the findings are derived from a computational simulation, so are estimates, not actual observations.Practical implicationsThe study challenged the business paradigm of the company the authors performed the study for. Now, it is implementing the suggested improvements.Originality/valueAn investigation into how a number of sawmilling companies working together can profit from their distinctive competitive advantages, and which proved successful in challenging one company's business paradigms.

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