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When trying to model the behavior inside the
business process we come to an difficult question:
which notation to use? Here we quote Albert
Einstein, who said: “Whether you can observe a
thing or not depends on the theory which you use.
It is the theory which decides what can be
observed.” Most often used swimlane type business
process models are not very intuitive in trying to
visualize behavior over time. A natural choice for
modeling the behavior of the flows inside the
business processes are Stocks and Flows models
which are known from the field of System Dynamics
(Forrester, 1961). The Stocks and Flows models of
the business processes are also suitable for computer
simulations. They help in understanding the
behavior of the stocks and flows over time.
Simulations also help in finding the critical points to
remove bottlenecks and improve the overall
performance of the processes. Chapter 2 of this
paper gives a short introduction in using stocks and
flows for modeling behavior over time of a simple
business process. Chapter 3 explains a real case
where stocks and flows models and simulations
helped us to reveal the underlying problems and find
improvements to the business processes. Chapter 4
discusses about the usefulness of stocks and flows
diagrams in business process modeling and chapter 5
draws the conclusions.
2 STOCKS AND FLOWS
The most basic visual model of a business process
according to Davenport is shown in figure 1.
The common method for modeling business
processes is based on the activities, the flow of work
from one task to next. Typical visual model is the
swimlane and its many variants, BPMN perhaps
Figure 1: Business Process.
most widely used. Business processes consist of
tasks and their sequence, logic for branching the
sequence based on rules and conditions and actor
who carry out the tasks. The swimlane represents all
the different paths what one process instance can
take. But when we are interested in behavior of the
aggregation of the business process instances over
time the swimlanes do not give much help. Input to
the process usually is not evenly distributed over
time. The flow of input items may have fluctuations
and variations, backlog of work may be
accumulating and other phenomena may occur that
have consequences on how the process will produce
its output.
To understand how the flow of input will affect
the output we turn the same business process model
into very high-level diagram of stocks and flows.
We start with identifying the first stock. The stock
represents an accumulation of some entity, so Input
waiting to be processed is the obvious choice for the
first stock. A stock is visualised with a rectangle,
which has a noun as a name. A stock is represented
by the value of a corresponding variable like number
of input items. The value represents always the size
of the stock in some point of time. That value can be
changed only by a flow. A flow can be an inflow,
which will increase the stock. A flow can also be an
outflow, which will decrease the stock. A flow is
visualised by an arrow and valve in the middle of it.
The value assigned to a flow represents the rate of
change of the respective stock over time, like
processed input items per hour. Figure 2 shows
previous diagram using stocks and flows notation.
Figure 2: Stocks and Flows Diagram.
In the diagram we have input items flowing in and
accumulating to wait for processing. The activity
“processing” will deplete the stock and move
finished items into accumulation of output items.
The flow input starts from a cloud and the flow
output ends in a cloud. Those are how we set
boundaries in our model and are not interested what
lies outside those clouds.
Stocks and Flows Diagrams are useful in
simulating the behavior over time of the model.
Flows can be affected by information of the level of
stocks and other factors, which create feedback
loops and can make the process behavior non-linear.
There is some literature about modeling the business
processes using swimlanes and Stocks and Flows
diagrams (An and Jeng, 2005). In the next chapter
we show a real life example of modeling using
Stocks and Flows diagrams and how simulation of
the model helped to understand the business process
problems, which were not easily seen from
swimlane models.
Input
Process
Output
UsingStocksandFlowsDiagramstoUnderstandBusinessProcessBehavior
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