Low Back Pain (LBP) is the most common cause of work loss after the ordinary cold, and it is the single greatest source of compensation payments. In the U.S., it is estimated that one million workers sustain a low back injury every year, and that 217 million work days are lost annually at a cost of 11 billion dollars for males aged 18-55 alone. In an effort to better understand how to control the economic impact of this disorder, a System Dynamics model is being developed. It is hoped that the model, by generating scenarios on the cost effectiveness of different interventions, will provide useful insight into specific policies to fun research addressing the causes of LBP disability.
This paper introduces a new linguistic dynamic simulation methodology, SLIN which deals with systems defined in either qualitative or quantitative terms. The simulation mechanisms proposed in SLIN include a set of logical rules and fuzzy set theory. An application of SLIN to Sado estuary showed its promise but also some of its present limitations. Future developments including an appropriate diagrammatic representation, a new linguistic simulation computer language, implementation in parallel computers and subsequent real-time multi-expert based simulation are also discussed.
This paper reviews techniques that may assist the system dynamics modeller in defining variables and functional relationships, parameter estimation, validation, sensitivity and policy analysis. The evaluation was made in the context of water resources management modeling effort for the Guadiana basin in Algarve and based on scientific, economic and operational criteria. In general, it was difficult to point out the most appropriate technique but rather recommend combinations of methods for each modeling stage.
This paper describes the importance of feedback loops included in a policy model constructed for the Office of Conservation of the Bonneville Power Administration (BPA). First there is the description of the region and the responsibilities for conservation planning at the BPA, and then a description of the purpose, structure, and use of the policy model. Several feedback loops involving customer response to higher electric rates are selected for our discussion of feedback. The system dynamics treatment of these feedback loops is contrasted with the treatment found in most electric utility planning models in the USA. The paper concludes with an assessment of whether the inclusion of feedback has been important in BPA's application of the model.
This paper discusses some dynamic effects of robot's introduction on a company in the electric appliances industry. Two key aspects are analysed. The effects on cash flow are explored first, the conclusion is reached that under certain conditions it could represent a controlling element that would slow down the rate of robots' introduction with the respect to the ideal rate suggested on the sole basis of economic convenience. The availability of skilled personnel is considered next. This availability increases through on the job training as more robots are installed. Under most circumstances, however, the availability of skilled technicians represents a controlling element that definitely slows down the introduction of robots. The effectiveness of training technicians therefore represents a variable of strategic importance.
This paper discusses the reasons why Systems' Dynamics models frequently encounter considerable difficulties in gaining acceptance and suggests several ways for overcoming this obstacle. Resistance to models within organizations is usually generated by one or several of the following causes: insufficient credibility of model's proponents, inability to grasp model's usefulness, cultural background, fear of losing power and negative previous experience with models. In the special case of models addressing issues of wide public interest suggestions are presented on how to plan a communications strategy designed to generate support for the model or for the conclusions derived with its help.
Many features are necessary in a behavioural model of household car ownership and usage patterns. A description is given of the features of conventional equilibrium-based models, followed by a discussion of the most important dynamic issues underlying travel choice. These issues include household travel and activity budgets; state-dependent factors such as information search, cognitive processes, habits, attitudes, and inertia; and the role of the household lifecycle as a choice catalyst. Recent dynamic modelling approaches are described, followed by a description of a system dynamics modelling approach which incorporates the dynamic hypotheses discussed throughout. Finally, a direction of research is laid out, in which the model can be used to simulate household panel data as a basis for hypothesis training.
Model behavior evaluation is an important component of System Dynamics (SD) model validation. SD methodology has often been criticized for its lack of quantitative/formal behavior evaluation tools. System Dynamicists have responded by stating the relative, subjective, qualitative, nature of model validation. We argue that using formal quantitative behavior tools is not inconsistent with a relativist, holistic philosophy of model validation. We suggest a multi-step, quantitative behavior evaluation procedure which focuses on individual pattern components of a composite behavior pattern. The procedure is relatively easy to apply and to interpret. We then test the performance of the procedure through a series of simulation experiments. The experimental results suggest that the multi-step procedure is appropriate for SD model behavior evaluation. The experiments also give us an idea of what the expected value and the variations of suggested quantitative tools are.
A fast, interactive, large-scale, data-base-oriented simulation language (LAMDA) for IBM compatible microcomputers has been developed for System Dynamics' applications. Sophisticated output features include high-resolution graphics, full report generation capabilities with textual explanation, and user defined screen menu options, An integrated sensitivity/confidence package allows parameters or structures to be evaluated as a function of time and as a function of all other parameters for their impact on model results. Confidence intervals are determined along with the time and circumstances where certain information is critical. An integrated statistical package can be used to estimate model parameters based on historical information or, in combination with the data base, used to test model hypotheses and statistical inferences. LAMDA can also interface with FORTRAN applications. Extensive dialogue capabilities allow the model builder to make the model user-friendly and fit the need/sophistication of the client.