On the use of lexicographic min cost flows in evacuation modeling

Building evacuation problems can be represented as dynamic network-flow problems [3]. The underlying network structure of a building evolves through time yielding a time-expanded network (a dynamic network). Usually in such evacuation problems involving time, more than one objective function is appropriate. For example, minimizing the total evacuation time and evacuating a portion of the building as early as possible are two such objectives. In this article we show that lexicographical optimization is applicable in handling such multiple objectives. Minimizing the total evacuation time while avoiding cyclic movements in a building and “priority evacuation” are treated as lexicographical min cost flow problems.     

The validity of assumptions underlying current uses of Lanchester attrition rates

This study examines critically the various assumptions, results, and concepts that exist to date in the literature and scientific community concerning the relationships among the Lanchester, stochastic Lanchester, and the general renewal models of combat. Many of the prevailing understandings are shown to be erroneous.     

Minimizing absolute and squared deviations of completion times with different earliness and tardiness penalties and a common due date

We consider a single-machine scheduling problem in which all jobs have the same due date and penalties are assessed for both early and late completion of jobs. However, earliness and tardiness are penalized at different rates. The scheduling objective is to minimize either the weighted sum of absolute deviations (WSAD) or the weighted sum of squared deviations (WSSD). For each objective we consider two versions of the problem. In the unconstrained version an increase in the due date does not yield any further decrease in the objective function. We present a constructive algorithm for the unconstrained WSAD problem and show that this problem is equivalent to the two-parallel, nonidentical machine, mean flow-time problem. For the unconstrained WSSD and the constrained WSAD and WSSD problems we propose implicit enumeration procedures based on several dominance conditions. We also report on our computational experience with the enumeration procedures.     

Variance of the number of units produced on a transfer line with buffer inventories during a period of length T

The transfer-line models in the literature are planning models rather than operational models. That is, they are very useful for planning or designing the transfer line, but are less useful for controlling the daily operation of the line. The performance measure used in these models is the efficiency of the line A. The expected number of units produced during a period of length T cycles is AT. In this article a procedure is presented for calculating the variance of the number of units produced by the transfer line during a period of length T cycles. These two performance measures can be used to construct an interval estimate for, say, the number of units produced during a shift. This interval estimate is an operational guide for the production manager.     

Evaluating the Spillover Effects of the Colombian Conflict in Ecuador

The Colombian civil war lasted for nearly six decades with approximately 10 percent of its population being displaced over the same period. The implications of this conflict have transgressed international boundaries. Countries such as Ecuador experienced an exodus of victims as well as an increase in the presence of armed groups, along with the proliferation of illegal businesses. Even though the internal social and economic consequences of the Colombian conflict have been documented in the literature, there is yet to be a study addressing these issues from the perspective of impacted neighboring countries. In this work, we contribute to the literature by evaluating whether the influx of asylum seekers and the increasing presence of armed groups in the bordering provinces of Ecuador have lead to an increase in violence among these provinces. We do not find any link between the arrival of asylum seekers and the incidence of violent crimes in the Ecuadorean bordering provinces. Similarly, our results indicate that despite an increase in the presence of armed groups, these regions did not experience an increase in the homicide rates significantly different from the other provinces. The results are robust to various specifications and econometric techniques.     

The effect of correlated exponential service times on single server tandem queues

An investigation via simulation of system performance of two stage queues in series (single server, first-come, first-served) under the assumption of correlated exponential service times indicates that the system's behavior is quite sensitive to departures from the traditional assumption of mutually independent service times, especially at higher utilizations. That service times at the various stages of a tandem queueing system for a given customer should be correlated is intuitively appealing and apparently not at all atypical. Since tandem queues occur frequently, e.g. production lines and the logistics therewith associated, it is incumbent on both the practitioner and the theoretician that they be aware of the marked effects that may be induced by correlated service times. For the case of infinite interstage storage, system performance is improved by positive correlation and impaired by negative correlation. This change in system performance is reversed however for zero interstage storage and depends on the value of the utilization rate for the case where interstage storage equals unity. The effect due to correlation is shown to be statistically significant using spectral analytic techniques. For correlation equal unity and infinite interstage storage, results are provided for two through twenty-five stages in series to suggest how adding stages affects system performance for ρ>0. In this extreme case of correlation, adding stages has an effect on system performance which depends markedly on the utilization rate. Recursive formulae for the waiting time per customer for the cases of zero, one, and infinite interstage storage are derived.     

The Iraq Survey Group: From Weapons of Mass Destruction to Counterinsurgency

The discovery and elimination of Iraq's alleged weapons of mass destruction (WMD) was a focal point of the Coalition's strategy and operations in the aftermath of Operation ‘Iraqi Freedom’. Despite the failure of the WMD mission to meet expectations after almost two years of intensive operations, the Iraq Survey Group (ISG) was able to integrate multinational and multi-agency forces to provide detailed intelligence regarding the history of Iraq's WMD programs and to assist in the battle against insurgents. The ISG's experience raises the question of whether a standing intelligence force could become a key component in future counterproliferation and counterterrorism efforts around the world.1 ¹The views expressed in this paper are those of the author and do not reflect the official policy or position of the Defense Intelligence Agency, the Dept. of Defense, or the US Government. View all notes     

Reviews

Ian Castle and Ian Knight, Fearful Hard Times: The Siege and Relief of Eshowe, 1879. Greenhill Books, London 1994, Pp.256, 85 illus, 6 maps, index. £17.95. ISBN 1–85367–180–0.

Michael E. Brown (ed.), Ethnic Conflict and International Security. Princeton: Princeton University Press, 1993, Pp.263, selected biblio, index. $45/£30 (cloth); $14.95/£12.95 (paper). ISBN 0–691–03368 and 00068–9.

Deborah Poole and Gerardo Renique, Peru: Time of Fear. London: Latin American Bureau (Research and Action) Ltd., 1992. Pp.212, 7 maps, photographs, glossary, sources/further reading, index. £17.99 ISBN 0–906156–70‐X and ‐71–8.

David A. Charters (ed.), Peacekeeping and the Challenge of Civil Conflict Resolution. Center for Conflict Studies, University of New Brunswick, 1994. Pp.viii+220. NP.

Joachim Remak, A Very Civil War: The Swiss Sonderbund War of 1847. Boulder, Colorado: Westview Press, 1993. Pp.xvi+221, 6 map's, 68 illus, biblio, index. $24.95/£16.95.