SIMULATION: Transactions of The Society for Modeling and Simulation International

Ahlgren, David J.; Stein, Alex C.

doi: 10.1177/003754979005400103pmid: N/A

This paper describes HIV transmission models built using STELLA, a new language for the Apple Macintosh. These include a basic model, or template, of sexual transmission of HIV; a model of heterosexual transmission constructed from the template; and a model which simulates transmission caused by needle sharing. The paper also describes a computer program, based on the template rate equations, which optimizes key epidemiological parameters associated with the early epidemic in San Francisco. With this program we have shown that the per contact transmission probability (infectivity) of the antigen- positive first stage of infection may be significantly higher than the infectivity of the antibody- positive subsequent stage. This finding stresses the importance of screening for viral antigens to detect first stage infection in high risk groups.

Virkkunen, Juhani; Hämäläinen, Raimo P.

doi: 10.1177/003754979005400104pmid: N/A

The basic dynamics of the AIDS epidemic in an homogeneous population is studied from three different points of view using simple mathematical models. The first model assumes a constant transmission probability per sexual intercourse and a constant frequency of sexual intercourses. Then the probabil ity for HIV infection strongly increases with increasing number of partners only if the infection probability per intercourse is relatively high.Second, the assumption that each carrier transmits the infection a constant number of times with a constant time interval between transmissions is shown to result in an exponential rise in the number of infected. Taking the saturation effect into account the number of new infections per year and the actual proportion of infected are studied in populations of different sizes.The third model assumes an exponen tial rise in the number of infected and a randomly distributed incubation time of AIDS. This results in stochastic simula tion of the rise in AIDS cases. With repeated simulation runs the stochastic dispersion of the rise is shown to be quite large obscuring the so-called transient phenomenon in the initial phase of the epidemic.

Kotva, Milan; Zdenková, B.

doi: 10.1177/003754979005400105pmid: N/A

It is not possible to ascertain directly the exact actual value of prevalence of HIV-infected persons without clinical symptoms of AIDS. Therefore a very simple simulation model was developed for the estimation of this value in the early stage of the epidemic. This model is based on the principles of the general method of multicompartmental models and its core is the so-called one-way transport segment which imitates very faithfully the supposed transition of HIV-infected persons through the stage from the transmission of infection to AIDS manifestation. The number of AIDS cases is represented in the model by a compartment. The essence of the estimation procedure is also very simple —it consists in the finding of such shape of the time-dependent parameter of exponential input function of the transport segment for which the course of the compartment measure values agree with the ascertained or prognosti cated data about the numbers of AIDS cases. The dependence of the exponen tial function parameter on the product of the average probability that the infection is transmitted from the infected to the receptive individual and of the average rate at which new sexual partners are required can be shown.

Banks, Jerry

doi: 10.1177/003754979005400106pmid: N/A

This paper describes a procedure used to develop verifi cation and validation techniques for very large simulation models. The procedure uses analogies from other systems to stimulate analogies which can then be implemented in the simulation environment.Sixteen systems were studied and some 64 analogies were developed. Several of the analogies are currently being considered for possible extension and application to complex simulation models.

Pruett, James M.; Vasudev, Vinay K.

doi: 10.1177/003754979005400107pmid: N/A

The key to the successful "factory of the future" will be the effective coordination of managerial functions and production operations. The simulation model described in this paper addresses this "coordination" issue, especially in terms of managing the impact of interrelated alternatives and activities in an information-rich environment. An evolutionary simulation modeling program, MOSES (Manufacturing Organization Simulation & Evaluation System), has been developed for the purpose of describing, analyzing, and understanding manufacturing organizations.MOSES is both a program and a concept. As a program, it is a menu-driven, interactive, discrete-simulation modeling tool. MOSES users are able to simulate any discrete-product manufacturing environment. As a concept, MOSES is a blend of simulation modeling and information-based management. It is structured around the idea that all manufacturing organization functions can be categorized into one of four areas—marketing, production, inventory, and accounting—and that organizational success depends on decisions made by managers of those areas. MOSES users manage the organization by observing, experimenting with, and modifying the simulation-driven activities. This user interaction may take place before, during, or after the simulation process.MOSES is evolutionary in several ways: (1) the user is able to build a meaningful manufacturing organization simulation model without writing any computer code; (2) the model spans the entire manufacturing organization, not just the production function; (3) the model includes optimization techniques and heuristics along with traditional logical modeling principles; (4) the model' stores and manipulates data using database technology rather than more traditional file techniques ; (5) the system includes the user as an integral part of the simulation process.