Stock and flow diagrams contain specific
symbols and components representing the structure of a system.
Stocks are things that can accumulatesuch
as employee head count or inventory. (Think of a stock as a bathtub.)
Flows represent rates of changesuch
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
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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