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STOCKS
AND FLOWS
Stock and flow diagrams contain specific
symbols and components representing the structure of a system.
Stocks are things that can accumulate—such
as employee head count or inventory. (Think of a stock as
a bathtub.) Flows represent rates of change—such
as annual employee turnover or quarterly reductions in inventory
through sales. (Think of a flow as a bathtub faucet, which
adds to the stock, or a bathtub drain, which reduces the stock.)
These diagrams also contain "clouds," which represent
the boundaries of the problem or system in question. Here's
a simple example:
Stock
and flow diagrams provide a bridge to simulation
modeling, because they help you assign equations to the
relationships between variables. Creating a stock and flow
diagram together with your team is valuable because it generates
as full a picture as possible of how everyone views the system
in question. Remember the parable about the blind men feeling
the elephant (the man feeling the trunk thinks of elephants
as long and skinny; the one feeling the ear thinks of the
animals as flat and floppy)? As this parable suggests, you
can design effective solutions to problems only after you
have as complete a picture as possible of what systemic structures
are causing your problem.
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