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System Dynamics Glossary

This is the System Dynamics Glossary compiled by David N. Ford and Accepted by Yaman Barlas. It was published on November 7th 2019 here: https://doi.org/10.1002/sdr.1641.

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Introduction

As a narrowly focused dictionary, this glossary defines commonly used terms that are central to traditional system dynamics and some more general terms that have special meanings or particular importance within system dynamics. General terms that have no special meanings in system dynamics, application‐specific terms, and software‐specific terms are not included. The reader is referred to the relevant literature for those definitions, as well as detailed descriptions, explanations, and examples of the terms included here.

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Click on the Concept to View the complete Description.

Concept
Description
Doubling time
The length of time it takes a quantity to double in size. Normally associated with exponential growth.
Dynamic
Changing over time. The opposite of static.
Dynamic hypothesis
A structure that the modeler advances to explain a dynamic behavior of interest.
Endogenous variable/view
Internal, the opposite of exogenous. An endogenous view approaches a problem searching for its causes and solutions within the system boundary. Endogenous variables are affected by other system variables.
Equilibrium
Conditions in a dynamic system where the inflows and outflows of each stock balance each other, and the sizes of the stocks do not change.
Equilibrium behavior
A behavior mode in which all stocks are at equilibrium conditions. Static equilibrium behavior occurs if all flows are zero (so the contents of stocks do not change over time). Dynamic equilibrium behavior occurs if flows are non‐zero but they balance (so the contents of stocks change, but their values stay constant) over time. Asymptotic equilibrium behavior means the system approaches equilibrium values, but does not reach these values in finite time.
Equilibrium conditions
A system structure and set of numeric conditions that generate equilibrium behavior.
Exogenous variable/view
External, the opposite of endogenous. An exogenous view assumes that a system's behavior is dominated by the influence of outside forces or factors. An exogenous variable is an external (input) variable that affects but is not affected by the system.
Exponential behavior
A nonlinear behavior mode generated by a relationship in which the change in a stock variable is proportional to the size of the variable itself.
Exponential decay
A behavior mode that occurs when the rate of increase or decrease in a variable (usually a stock) is proportional to how far the stock is from its equilibrium, so as to slow down its rate of change. As the stock gets larger (smaller), its increase (decrease) occurs progressively more slowly. The speed of increase or decrease can be described by half‐life. The corresponding structure is associated with negative feedback and tends to generate goal‐seeking behavior.
Exponential delay
A model structure in which a value moves towards the input or target value gradually, in a goal‐seeking exponential fashion.
Exponential growth (or collapse)
A behavior mode that occurs when the rate of increase or decrease in a stock variable is proportional to the size of the stock at that point in time, so as to accelerate its change. As the stock gets larger (smaller), its increase (decrease) occurs progressively more quickly. The speed of increase or decrease can be described by doubling time. The corresponding structure is associated with positive feedback.

References

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