Salah E. Elmaghraby

Awards & Honors

Dr. Elmaghraby is a recipient of the Frank and Lillian Gilbreth Industrial Engineering Award, IIE, 2003, the Alexander Quarles Holladay Medal for Excellence, NCSU, 2000; Honorary Doctorate, University Claude Bernard Lyon I, Lyon, France, October 1998; Visiting Scholar at the FUCAM, Mons, Belgium, summers of 1998 through 2003; invited as The Thomson Chair Professor of Production Management, Claude Bernard University of Lyon I, Lyon, France, the summers of 1991, 1992, 1995; recipient of the Kuwait Foundation for the Advancement of Science Distinguished Award, May 1990; the R.J. Reynolds Distinguished Award in Research and Education, College of Engineering, NCSU, 1987; the Operations Research Division Award, IIE, 1980; the David F. Baker Distinguished Research Award, IIE, 1970; First Prize, National Center for Education & Research in Equip. Policy, 1958; First Prize, Morse Chain Co. Competition Award, Ithaca, NY, 1957. Dr. Elmaghraby was elected Fellow of the Institute of Industrial Engineers, 1986.

Research Interests

Production Planning in all its aspects, especially capacity planning and scheduling; Project Planning and Control; and the application of Dynamic Programming to operational systems.

Most Recent Publications

1. “Optimal start times under stochastic activity durations,” (2000), IJPE 64, 153-164, with A.A. Ferreira and L.V. Tavares.
   
2. "On criticality and feasibility in project networks," (2000) EJOR 127, 220-238.
   
3. “Risk analysis in activity networks: Resource allocation and budget estimation,” (2000), Invited Plenary Session Talk, PMS011, Istanbul, Turkey, and appearing as extended abstract in the Proceedings of the conference.
   
4. “Simple heuristics for the two machine openshop problem with blocking,” (2000), J. Chinese Inst. of Ind. Eng. 17, 537-547, joint with M-J. Yao and H. Soewandi.
   
5. “On computing the distribution function of the sum of independent random variables,” (2000), Comp. and Oper. Res. 28, 473-471, with M.K. Agrawal.
   
6. “Chance-constrained programming (CCP) in activity networks: A critical evaluation,” (2001) Euro. J. Oper. Res. 131, 440-458, with H. Soewandi and M.J. Yao.
   
7. “The economic lot scheduling problem under power-of-two policy,” (2001) Comp.& Math.Appl. 41,1379-1393.
   
8. “Sequencing jobs on two-stage hybrid flowshop with identical machines to minimize makespan,” (2001), IIE Trans.33, 985-993; with H. Soewandi.
   
9. “On the optimal release time of jobs with random processing times, with extensions to other criteria,” (2001), IJPE 74, 103-113.
   
10. “Scheduling hybrid flowshops in printed circuit board assembly lines,” (2002), POM 11. 216-230, with Z.H. Jin, K. Ohno, and T. Ito.
    
11. “Sequencing to minimize the weighted completion time subject to ‘dedicated’ resources and arbitrary precedence,” presented at PMS’02, Valencia, Spain, Apr.3-5, (2002), with S. Balisetti, and appearing in extended abstract form in the Proceedings of the conference.
    
12. “Sequencing precedence related jobs on parallel machines to minimize the weighted completion time,” presented at ISS’02, Hamanako, Japan, June.4-6, (2002), with Girish Ramachandra, and appearing in extended abstract form in the Proceedings of the conference.
    
13. “On the Feasibility Testing of the Economic Lot Scheduling Problem Using the Extended Basic Period Approach,” J. Chinese Inst. Ind. Eng. 13, 13-20 (2002). Co-authored with M-J. Yao and I-C. Chen.
    
14. “Note on the paper ‘Resource-constrained project management using enhanced theory of constraint’ by Wei et al.”, Inter’l J. Project Management 21 (2003), 301-305, with W.S. Herroelen and R. Leus.
    
15. “Sequencing on two-stage flowshops with uniform machines to minimize makespan,” (2003), IE Trans. 35(5), 467-478, with H. Soewandi.
    
16. "Adaptive Resource Allocation in Multimodal Activity Networks,” (2003) accepted for publication, IJPE, with A.P. Tereso and M.M.T. Araújo.
    
17. “Scheduling jobs related by precedence on one machine to minimize the sum of weighted completion times,” (2003) submitted for publication, J. Scheduling.

Abstracts of Selected Publications

"On Criticality and Sensitivity in Activity Networks," Keynote Address, PMS Istanbul, Turkey, July 11-15 (1998), to appear IJPE.
A review of the issues related to activity criticality and the sensitiviey of the mean and variance of project compleiton time to changes in the mean and variance of individual activities. The methodologies proposed range over the analytical, Monte Carlo sampling, and statistical sampling, in particular the use of  Taguchi orthogonal arrays.

"Chance-Constrained Programing in Activity Networks: A Critical Evaluation," Paper presented at PMS Istanbul, Turkey, July 11-15 (1998), submitted to EJOR, co-authored with H. Soewandi and M-J. Yao.
We review the chance-constrained programming (CCP) model from the points of view of accuracy and validity. Our analysis reveals that the CCP model results in gross errors in yielding a point estimate for the project completion time that satisfies a required confidence level, and that it is of little help in determining the cumulative distribution function (cdf) of the project completion time, as suggested by some. We show that the CCP model leads to an extremely weak lower bound on the cdf. A backtrcking approach is proposed that improves the CCP lower bound, but the improvement is marginal, at best. Finally, we utilize the CCP model to derive a result due to Kress in a much simpler and more direct way. However, we also demonstrate that the result itself is of questionable validity.

"Optimal Start Times Under Stochastic Activity Durations," Paper presented at IEPM Lyon, France, October 20-24 (1997), to appear in IJPE; co-authored with A.A. Ferreira and L.V. Tavares.
We treat the problem of sequencing n activities on a single facility with the objective of determining their start times to maximize an economic gain, subject to restricitons on their availabilities. The activities possess random durations. We present a dynamic programming model that determines the optimal value and yields the desired start times, which also imply the optimal sequence. An approximation is also offered to lighten the computing burden somewhat.

"An Optimal Assembly Mode of Multi-Type Printed Circuit Boards", co-authored with K. Ohno and Z. Jin, Computers And Ind. Eng. 36, (1999), 451-471.
We deal with the problem of assembling several types of PCBs on a machine with multiple pick-insertion heads. We partition the PCB types into subsets, which constitute the modes of operation. The subsets are selected so that the components required for assembly on the PCBs in a subset fit within the limited capacity of the reel carrier. Each PCB type in the subset is assembled successively lot-by-lot without setup between the lots. Setup is needed only in the changeover between subsets. An optimal assembly mode minimizes the sum of assembly times and setup times of all PCB types demanded. Our approach is to divide the overall problem into three sub-problems: an insertion sequence problem (ISP), a reel positioning problem (RPP), and an optimal assembly mode problem (OAMP). The ISP for each type of PCB is formulated as a traveling salesperson problem for a fixed reel positioning. The RPP is formulated as an assignment problem for which the assignment cost is the sum of the weighted tour costs of the traveling salesperson problems for the subsets of PCB types. The ISPs and RPP are solved by a heuristic algorithm based on the two-optimal local search heuristic for the traveling salesperson problem, and an evolution strategy for the RPP. The OAMP is formulated as a set partitioning problem with added traveling salesperson type constraints. The proposed algorithm was implemented on a real life problem, and the optimal assembly mode was determined.
 
"On the Sensitivity of Project Variability to Activity Mean Duration", co-authored with Y. Fathi and M.R. Taner , IJPE 62, (1999), 219-232.
Traditionally the ‘importance’ of an activity in the PERT model of projects is measured by its ‘criticality index', which is defined as the probability that the activity will be on the longest path. An insightful discussion by Williams (1992, Journal of Operational Research Society 43, 353--357) revealed that the classical criticality index is not always informative or intuitively appealing. In a recent study by Cho and Yum (1997, International Journal of Production Research 35, 2737--2757), a new `Uncertainty Importance Measure' is defined to measure the effect of the variability in an activity duration on the variability of the overall project duration. They propose Taguchi's sampling technique as a method for analyzing the network. The main contribution of this paper is to study the impact of changing the mean duration of an activity on the variability of the project duration. On the way to accomplish this, we further investigate the accuracy of Taguchi's sampling method for approximating the mean and standard deviation of the project duration, and propose steps that could result in computational savings in large networks.  

"The two-machine stochastic flowshop problem with arbitrary processing time distributions", co-authored with Kristin A. Thoney, IIE transactions 31, (1999), 467-477.
We treat the two-machine flowshop problem with the objective of minimizing the expected makespan when the jobs possess stochastic durations of arbitrary distributions. We make three contributions in this paper: (1)we propose an exact approach with exponential worst-case time complexity. We also propose approximations which are computationally modest in their requirements. Experimental results indicate that our procedure is within less than 1% of the optimum; and (2) we provide a more elementary proof of the bounds on the project completion time based on the concepts of ‘control networks’; and (3) we extend the ‘reverse search’ procedure of Avis and Fukuda [1] to the context of permutation schedules.  

"Call Admission Control Schemes and ATM Network Topological Design", co-authored with S.Z. Lo, B. Makrucki and G. Bilbro, Euro. J. Operl Res. 111, (1998),.393-404.
Call admission control criteria are not only important for call admission control itself, but also can be an important input to network topological design. In this paper, we show the difference in terms of network cost incurred by adopting different call admission control schemes in network topological design. We compare two call admission control schemes. Scheme 1 uses equivalent bandwidth as its call admission control criterion and Scheme 2 is based on modeling the volatility of call traffic using Reflected Brownian Motion. Though Scheme 2 increases the complexity of network topological design, it can give lower network costs. Our experimental results show that for the same traffic mix, the network cost can be as little as 10% and as much as 35% lower when Scheme 2 is used instead of Scheme 1. The differences between the pair of resulting networks suggests that network topological design can be used as one of the criteria for choosing the call admission control scheme.

"A Hybrid Three-Stage Flowshop Problem: Efficient Heuristics to Minimize Makespan", co-authored with F. Riane and A. Artiba, Euro. J. Operl Research 109, (1998), 321-329.
We treat a problem of scheduling n jobs on a three stages hybrid flowshop of particular structure (one machine in the first and third stages and two dedicated machines in stage two). The objective is to minimize the makespan. This problem is NP-complete. We propose two heuristic procedures to cope with realistic problems. Extensive experimentation with various problem sizes are conducted and the computational results show excellent performance of the proposed heuristics.  

"DAGEN: A Generator of Testsets for Project Activity Nets", co-authored with M.K. Agrawal and W.S. Herroelen, Euro. J. Operl Res.90, (1996), 376-382.
We offer a methodology, and the relevant software, to generate source--terminal directed project nets of specified number of nodes, number of activities, complexity index, and ranges of costs and resources utilization.

"Optimal Procedures for the Discrete Time/Cost Trade-Off Problem in Project Networks", co-authored with E.L. Demeulemeester and W.S. Herroelen, Euro. J. Operl Res.88, (1996), 50-68.
We describe two algorithms, based on dynamic programming logic, for optimally solving the discrete time/cost trade-off problem (DTCTP) in deterministic activity-on-arc networks of the CPM type, where the duration of each activity is a discrete, nonincreasing function of the amount of a single nonrenewable resource committed to it. The first algorithm is based on a procedure proposed by Bein, Kamburowski and Stallmann for finding the minimal number of reductions necessary to transform a general network to a series--parallel network. The second algorithm minimizes the estimated number of possibilities that need to be considered during the solution procedure. Both procedures have been programmed in C and tested on a large set of representative networks to give a good indication of their performance, and indicate the circumstances in which either algorithm performs best.