OR/MS Today - October 2000

October 2000

In Memoriam
A. Alan B. Pritsker
(1933-2000)

By James R. Wilson and David Goldsman

Alan Pritsker, one of the founders of the field of computer simulation, passed away on August 24. Over the course of his 45-year career, Alan made seminal contributions to many areas of the field of simulation and to the larger fields of operations research and industrial engineering. Particularly notable is the extent to which Alan's accomplishments spanned both the theory and practice of engineering and science. Alan was always keenly interested in bringing the latest research results quickly into practical applications for the benefit of society at large. From his pioneering work in developing theory and methodology for modeling and analysis of large-scale industrial systems to his leadership in developing commercial simulation software products and crafting policy analyses to address major societal problems, Alan's multifaceted contributions to the engineering profession are striking not only for their scope and impact but also for the remarkably long time period over which that impact has been sustained in all its dimensions.

Comprehensive documentation of Alan's career can be found in his professional autobiography, "Papers, Experiences, Perspectives" (Systems Publishing Corporation 1990). Moreover, in March 2001, the Institute of Industrial Engineers (IIE) will publish a special issue of its flagship journal IIE Transactions to honor Alan for his contributions to the engineering profession. With a lead article titled "Alan Pritsker's Multifaceted Career: Theory, Practice, Education, Entrepreneurship, and Service," this is the first issue of IIE Transactions to so honor an individual.

Foremost among Alan's achievements is his work in the theory and methodology of discrete and combined discrete-continuous system simulation. During the early 1970s, Alan and his students formulated the basic principles of combined discrete-continuous simulation and implemented those principles in the GASP IV, SAINT and SMOOTH simulation languages. Subsequently, Alan extended the foundations of combined simulation to encompass the process-interaction approach. Working with several collaborators, he implemented a family of simulation software systems, including SLAM and its extensions — SLAM II, TESS, SLAMSYSTEM, FACTOR/AIM and Visual SLAM/AweSim. Alan's pioneering work on combined simulation forms the basis for virtually all of the major combined simulation languages in use today. His strategic vision of a family of related simulation software products played a crucial role in the growth and maturing of the field of simulation over the past 30 years.

Alan also made numerous fundamental contributions to the theory and methodology for analysis of stochastic networks and more general simulation experiments, particularly in the following areas: the spectral method for simulation output analysis; the simulation start-up problem; variance reduction techniques for the simulation of queueing systems; and analytic characterizations of the transient and steady-state behavior of the variance of the sample mean for simulated Markov processes in discrete and continuous time. The Graphical Evaluation and Review Technique was a breakthrough that provided a method to calculate the moment-generating function of the accumulated time (or cost) to realize a stochastic network composed of "exclusive-or" nodes and arcs whose traversal times (costs) are independent random variables. For this work, Alan and Gary Whitehouse received the "H. B. Maynard Innovative Achievement in Industrial Engineering Award" from the American Institute of Industrial Engineers (AIIE, now IIE) in 1978. For work on variance reduction techniques tailored to queueing simulations, Alan and Jim Wilson received the "Outstanding Simulation Publication Award" from the TIMS-College on Simulation and Gaming (now the INFORMS-College on Simulation, or INFORMS-CS) in 1985. Alan's most recent research focused on the formulation and implementation of practical, efficient statistical screening, selection and multiple-comparison procedures that are adapted to large-scale system simulation. Until his untimely death, Alan had frequent telephone conversations with his collaborators David Goldsman and Barry Nelson about this line of research.

Most of Alan's work in theory and methodology was driven by real-world problems arising in operations research and industrial engineering. Perhaps Alan's most prominent contribution to engineering practice was his recent work on policy analysis for organ transplantation. From 1995 to 1999, Alan led the development and use of large-scale simulation models of various operations of the United Network for Organ Sharing (UNOS). In particular, the UNOS Liver Allocation Model (ULAM) has been used to compare proposed policies for allocating donated livers to patients who are waiting for a liver transplant. Partially on the basis of Alan's extensive analyses of ULAM-generated predictions of the effects of implementing various organ-allocation policies, the UNOS Board of Directors made a highly publicized change to the UNOS liver-allocation policy on Jan. 20, 1997. Moreover, on June 18, 1998, Alan testified in Congressional Hearings on the results of his comparison of the current liver-allocation policy vs. a policy based on a sickest-patient-first national waiting list. Shortly thereafter, Congress authorized the continuation of the current policy for a year pending an independent review of this policy by the Institute of Medicine of the National Academy of Sciences. This is a remarkable example of the definitive practice of system simulation in addressing ultimate questions of life and death. A more complete discussion of Alan's work on policy analysis for organ transplantation can be found in his article titled "Life & death decisions: Organ transplantation allocation policy analysis" that appeared in OR/MS Today (1998, vol. 25, no. 4, pp. 22-28).

Alan's first love was teaching — not only undergraduate- and graduate-level university courses but also professional short courses. Alan served on the faculties of Arizona State University (1962-69), Virginia Polytechnic Institute and State University (1969-70) and Purdue University (1970-98). From 1970 to 1973, he served as director of the Center for Large-Scale Systems at Purdue University. During the 1970s and 1980s, Alan's activities at Purdue led to what many have called the "Golden Age of Simulation." He received honorary doctorates from Arizona State University (1992) and Purdue University (1998).

Alan's groundbreaking contributions to engineering theory, methodology and practice are widely available via numerous well-known texts and book chapters that he wrote over the past 45 years. Perhaps more than any other single individual, Alan effectively disseminated knowledge about simulation technology at all levels of academia, government and industry through the publication of twelve popular textbooks. Alan received Honorable Mention in the 1980 Lanchester Prize Competition of the Operations Research Society of America (now INFORMS) for three of his books: "The GASP IV Simulation Language" (1974); "Modeling and Analysis Using Q-GERT Networks" (second edition, 1979); and "Introduction to Simulation and SLAM" (1979). Over the past three decades, Alan also wrote more than 15 book chapters on simulation and its applications to manufacturing, production scheduling, computer science and ergonomics.

In addition to educating undergraduate-level students in hundreds of traditional academic courses and industrial short courses on simulation, Alan compiled a superlative record as an adviser of graduate students. Of the 18 doctoral students and more than 60 master's students who completed their graduate work under Alan's supervision, virtually all are highly successful professionals in academia, government or industry. George Fishman of the University of North Carolina at Chapel Hill offered his perspective on Alan's work as an educator:

"More than any of his other contributions, there is one that I regard as his greatest accomplishment, and the one that other teachers of simulation can only envy. That is the great number of students that Alan has interested in simulation and who have gone on to successful careers using this methodology either in academia or industry. No one among us in the simulation community comes close to Alan in this regard."

Another important aspect of Alan's contributions to the growth of simulation was his role in founding and leading several commercial enterprises dedicated to the development and dissemination of simulation technology. He was a cofounder of Pritsker & Associates, Inc. (1973). He also served as the Board chair of FACTROL, Inc. (1986-89). When Pritsker Corporation was created in 1989 through the merger of Pritsker & Associates and FACTROL, Alan served the new company as Board chair and CEO (1989-91, 1996-98) and as president and CEO (1991-96). In 1987, Alan received the "Arthur Young—VENTURE Magazine Entrepreneur of the Year Award."

Alan's service to the profession spanned a broad range of activities sustained over the past four decades. Perhaps Alan's most prominent service contributions were made through his leadership of the Winter Simulation Conference (WSC). He served as a member of the WSC Board of Directors representing AIIE from 1970 to 1973. He also served on the WSC Board of Directors representing TIMS-College on Simulation and Gaming from 1981 to 1987; and he served as Board Chair from 1984 to 1985. Alan was an active participant in the technical program of the WSC each year for over 30 years; in 1989 he delivered the keynote address for that conference.

Through leadership in various professional societies and governmental organizations over the past 45 years, Alan contributed significantly to the dramatic growth of the field of simulation as well as to operations research and industrial engineering. He co-founded the Operations Research Division of AIIE in 1968, and he served as the director of that division from 1968 to 1970. He also cooriginated the AIIE Systems Engineering Conference in 1973. He received the "AIIE Distinguished Research Award" in 1966, and he was elected a Fellow of AIIE in 1978. It is especially noteworthy that in 1991, Alan received from IIE the "Frank and Lillian Gilbreth Industrial Engineering Award," the highest and most esteemed honor presented by that organization.

Alan's service to professional societies was not limited to IIE. From 1973 to 1979, he served the Society for Computer Simulation as the associate editor for combined discrete-continuous simulation for the journal SIMULATION. For his long-standing, exceptional service to the international simulation community, Alan received the "Distinguished Service Award" from INFORMS-CS in 1991. Moreover, in 1999 Alan received the INFORMS-CS "Lifetime Professional Achievement Award," which is the highest honor given by that society. The award recognizes major contributions to the field of computer simulation that are sustained over a professional career, with the critical consideration being the total impact of those contributions on the field. An individual's contributions may fall in one or more of the following areas: research, practice, dissemination of knowledge, development of software or hardware, service, and advancement of the status of the field. In the award citation, the selection committee stated, "Alan is that rare individual who could have won the award based on his contributions in any of the areas, let alone all six!"

Elected to the National Academy of Engineering (NAE) in 1985, Alan enjoyed the distinction of being the second industrial engineer to join that organization. Over the past 15 years, Alan actively served NAE in positions of great responsibility. He was elected chair of the Industrial, Manufacturing, and Operational Systems Engineering Section for the period 1996-98; and in 1997 was a member of the Nominating Committee for President of NAE.

In the course of doing background research for the lead article on Alan's career that will appear in the March 2001 special issue of IIE Transactions, we amassed numerous statements from distinguished individuals who could evaluate authoritatively various facets of Alan's remarkable professional achievements. A common theme running through many of these statements is the extraordinary effect that Alan had on all his students, colleagues and friends. Another dominant theme is the exceptional scope and depth of Alan's professional activities. Lee Schruben of the University of California, Berkeley, tied these themes together neatly:

"Alan is directly responsible for the success of many careers in simulation, mine included. Indeed, he was the first, and maybe the last, to demonstrate that it is possible to be a first-rate practicing engineer, scholarly researcher, devoted teacher, and successful entrepreneur. Many people in the field have modeled their careers after one of these characteristics of Alan — none, to date, have succeeded in doing all four. Almost everyone in our field has been influenced at least indirectly by Alan's activities."

John White, Chancellor of the University of Arkansas, succinctly summarized Alan's contributions to the field of simulation with the following memorable comparison:

"I believe Alan Pritsker's accomplishments surpass those of, arguably, the world's greatest hockey player, Wayne Gretzky. It is said that good hockey players skate to where the puck is, but Gretzky skates to where the puck is going to be. Alan Pritsker did not just skate to where simulation was going to be; instead he took it to where it needed to be. He has shaped and defined the field."

From a larger perspective, Alan exerted the same superlative leadership in all aspects of his work in the fields of operations research and industrial engineering. We are indeed fortunate to have benefited from Alan Pritsker's multifaceted contributions to the life of our profession over the past five decades. He will be sorely missed by his family, friends and colleagues. Alan is survived by his wife, Anne; his children, Pam, Caryl, Ken and Jeff; and five grandchildren.

James R. Wilson is professor and head of the Department of Industrial Engineering at North Carolina State University. He did his graduate work at Purdue University under Alan Pritsker's supervision. His e-mail address is jwilson@eos.ncsu.edu, and his web address is www.ie.ncsu.edu/jwilson.

David Goldsman is a professor in the School of Industrial and Systems Engineering at Georgia Tech. His e-mail address is sman@isye.gatech.edu.

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