Element used to identify causal loops in
System Dynamics models.
Loop is just a graphical
identifier, consisting of a label with a brief description of the loop
meaning and an arrow showing the loop direction. It does not define the
causal loop itself, but only shows information about existing
causal dependencies in your stock and flow diagram, i.e. shows how
variables affect one another.
Adding loops, you can help other users of this model to understand
influences and causal dependencies that exist in the
stock and flow diagram.
Reinforcing and balancing loops
Loops are either reinforcing
To determine if a causal loop is reinforcing or balancing, one
start with an assumption, e.g. "VariableN increases" and follow the
The loop is:
if, after going around the loop, one ends up with the same result as
the initial assumption.
- balancing if
the result contradicts the initial assumption.
Or you can use the alternative definition:
loops have an even number of negative links (zero also is even)
loops have an uneven number of negative links.
There are two loops in the diagram shown in the figure below: one
reinforcing and one balancing:
- Depletion loop is
balancing. Starting with an assumption that SusceptiblePopulation grows
and going around the loop counterclockwise (according to the
direction of the link going from SusceptiblePopulation
to InfectionRate), we
get growth of InfectionRate,
but at the same time this leads to decrease of SusceptiblePopulation,
so the result contradicts the initial assumption. Alternative check of
number of negative links: in this loop we have one negative link -
the flow link that decreases the stock value, so again this loop is
- Contagion loop is
reinforcing. Let`s go around the loop clockwise. The increase of InfectiousPopulation leads to
increase of InfectionRate, and
that by-turn leads to greater increase of InfectiousPopulation.
Both links in this loop are positive.
add a loop
- Drag the Loop element from the System Dynamics
palette to the graphical diagram of the agent.
- Go to the Properties
view to set up properties of the loop.
- Set the Direction
of this loop - either Clockwise
- Choose the symbol that you
want to be shown for this loop from the Type
group of buttons. You can choose B
(stands for Balancing), R (Reinforcing),
specify your own text in the edit box to the right. We recommend you to
number your loops (R1,
B1, R2, etc.) to simplify referencing them by the
users of the model.
- You can specify a brief description of the loop (usually - two or
three words) that will be shown on presentation in the Text property.
- Optionally you can type more detailed description of this
loop on the Description property
(but it will not be visible on the presentation, users will be able to
see it only on this particular property page and in the model documentation).
– Here you can specify the direction of the loop - whether it
– Here you can choose the symbol that will be shown
by the loop element. You can choose B
(stands for Balancing), R
(Reinforcing), or specify your own text in the edit box to the right.
– The color of the loop
(both the text label and the arrow).
By default loops are shown blue. However, you can change the
default color to any other you want.
– Here you specify a brief description of the loop that will
be shown on presentation.