In
the above example, the stock shows the number of platforms that
are available for duty. This is determined by the rate at which
new platforms are introduced and the rate that platforms are
retired. The actions of these rates over time determines the
profile for the number of platforms that are available.
The
models developed using Powersim Constructor have a number of
similarities with models developed using spreadsheets. People
with experience of developing spreadsheets will find the majority
of the equations to be found in a Powersim model familiar. Indeed,
Microsoft Excel could be used to develop a system dynamics model
using data and formulae in cells. However, Powersim has a number
of features that makes the process of building system dynamics
models easier and more robust.
Each element in a stock-flow could be considered as a cell in
a spreadsheet in that it contains a number or it contains an
equation that is used to calculate a number. Equations in a
spreadsheet usually refer to other cells as variables in the
equations. Similarly, in a Powersim model the equations will
usually refer to other elements in the stock-flow diagram as
variables. The difference is that Powersim graphically shows
the developer where values are being referred to by using a
connector arrow.
A Powersim model uses the stock-flow diagram and its associated
data and equations to perform calculations repeatedly to represent
equal steps through time. The values that were calculated for
the stocks in the previous time period and the data form the
starting point for the calculations in the current period, which
in turn leads to updating the values in the stocks.
