(a) the level variables to characterise the accumulations, or integrations and
(b) the rate variables to denote the instantaneous flow rates.
The first set of variables is known as the level (or state) variables. Integrations
may be used if there are continuous flows, while summations are used if the flows
occur at distinct instants in time. The second set of variables is known as the rate
(or policy, or flow) variables. In contrast to physical systems, where the natural laws
govern how rate variables behave, in man-managed systems (infrastructure, socioeconomic or industrial), the rate variables often vary according to the policies behind
individual decisions. (see Table 6.3).
In fact, the hardest stages in system dynamics modelling relate to:
(a) monitoring the policies being applied to the existing system, and
(b) designing a better or new policy for the system.
They are hard because repeated experimentation with actual social systems is
both unwelcome and often impossible including the prospect of disastrous consequences, and a long period of time before reaching any conclusion.
Thus, a core problem in system dynamics modelling resides in precisely defining
the rates or the policy variables.
Physical flows are conserved flows. When an outflow takes place from one level
into another level, the volume stocked in the first level decreases while that in the
second level increases such that the total volume in both stocks remains the same.
Consider a source of large capacity from which a flow originates to end up in a sink,
then the total amount in the flow line, that is the sum of the amount stored in the
levels, the source, and the sink remains the same.
Usually, the rate variables are complex functions of level variables. For example,
one subdivides the rate into various auxiliary variables. Rate variables (or auxiliary
variables when they are used) may also depend on some constant terms within the
time frame considered. These constants must relate to real-life and must be distinguishable in real systems.
6.5.3 Information Flow
Information helps in decision-making. Therefore, rate variables are governed by
information about level variables and constants, and about auxiliary variables, if they
are defined. But given that auxiliary variables are subdivisions of rate variables, they
must appear only in the information flow channels linking the rates to the levels.
Information flows are not conserved flows. Information neither decreases the
value of the level, auxiliary variable or the constant from which the information is
abstracted, nor intensifies the value of the rate/auxiliary variable at which the
information is delivered, or used.
Rates always designate non-quantifiable instantaneous values; hence information
on rates should not be available. Therefore, no information should be abstracted
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(b) the rate variables to denote the instantaneous flow rates.
The first set of variables is known as the level (or state) variables. Integrations
may be used if there are continuous flows, while summations are used if the flows
occur at distinct instants in time. The second set of variables is known as the rate
(or policy, or flow) variables. In contrast to physical systems, where the natural laws
govern how rate variables behave, in man-managed systems (infrastructure, socioeconomic or industrial), the rate variables often vary according to the policies behind
individual decisions. (see Table 6.3).
In fact, the hardest stages in system dynamics modelling relate to:
(a) monitoring the policies being applied to the existing system, and
(b) designing a better or new policy for the system.
They are hard because repeated experimentation with actual social systems is
both unwelcome and often impossible including the prospect of disastrous consequences, and a long period of time before reaching any conclusion.
Thus, a core problem in system dynamics modelling resides in precisely defining
the rates or the policy variables.
Physical flows are conserved flows. When an outflow takes place from one level
into another level, the volume stocked in the first level decreases while that in the
second level increases such that the total volume in both stocks remains the same.
Consider a source of large capacity from which a flow originates to end up in a sink,
then the total amount in the flow line, that is the sum of the amount stored in the
levels, the source, and the sink remains the same.
Usually, the rate variables are complex functions of level variables. For example,
one subdivides the rate into various auxiliary variables. Rate variables (or auxiliary
variables when they are used) may also depend on some constant terms within the
time frame considered. These constants must relate to real-life and must be distinguishable in real systems.
6.5.3 Information Flow
Information helps in decision-making. Therefore, rate variables are governed by
information about level variables and constants, and about auxiliary variables, if they
are defined. But given that auxiliary variables are subdivisions of rate variables, they
must appear only in the information flow channels linking the rates to the levels.
Information flows are not conserved flows. Information neither decreases the
value of the level, auxiliary variable or the constant from which the information is
abstracted, nor intensifies the value of the rate/auxiliary variable at which the
information is delivered, or used.
Rates always designate non-quantifiable instantaneous values; hence information
on rates should not be available. Therefore, no information should be abstracted
176
6 Infrastructure as a System