STOCK AND FLOW DIAGRAMS

A SYSTEMS THINKING TOOL
By: Alan Ticotsky  |  2/14/2014

The world can be viewed through a variety of lenses.  When we look from the perspective of art, we may first notice color, shape, and other attributes that please us or evoke memories and emotion.  As a scientist, we can ponder molecular structure, organisms' roles in ecosystems, or estimate quantitative measures of what we see.  When we think as a writer or historian, we might try to read into the stories behind the scenes we observe.   As teachers, we are familiar with helping our students use these and many other perspectives as they learn subject after subject and travel from one class to another.

Systems thinking can be a unifying theme amid this complexity and shifting frames of reference.  The same tools, like behavior over time graphs and feedback loops, can be used across disciplines.  For example, the iterative process of revision and improvement based on feedback is common to virtually all academic subjects, including both the writing process and the design cycle.  Guiding students to recognize these generic traits of thinking and working can help us all become better at interdisciplinary learning.  Take a look at the "Habits of a System Thinker" - you will recognize most of the habits as core values  or desired behaviors in your life.

Another universal principle of systems thinking involves stocks and flows.  Stocks are accumulations of things, and flows measure the rates at which the stocks change.  In a similar way to the introduction of other tools, simple examples of stocks and flows may not add much insight to our understanding.  Water in a bathtub is a common example of a stock.  The flow from the faucet is an inflow that fills the tub, while the drain is an outflow to empty it.  The software named STELLA that we use at IACS draws clear maps of stocks and flows:

No big insight there.  The clouds on the ends of the flows mark the model boundaries.  While it's important where water comes from to fill the tub and where it travels when it drains, we are not modeling that larger system in our diagram.  This is a simple bathtub map.

But stock and flow thinking becomes difficult for us when the rates of change are confusing.  Let's substitute other names for the elements in our diagram:


Most people agree this causes a big problem, climate change.  Projections of energy use point out how stubborn the inflow is.  We are essentially running the faucet with the drain open, but the inflow is outpacing the outflow.  If we level off our CO2 emissions somewhere close to current levels (a giant challenge), the inflow will still exceed the outflow.  And when inflow > outflow, the stock increases.  That's true for water in a tub, CO2 in the atmosphere, organisms in a population, happiness in our lives ... anything we can represent as a stock and flow.  To reduce the level of CO2 currently in the atmosphere, the rate of removal must exceed the rate of additional CO2 being produced.

Here's one more stock and flow map to consider:

If we spend more than we can pay, our stock of debt rises.  But I suspect you know this isn't the whole story, especially if you've borrowed money.  Debt costs us money in the form of interest, and STELLA uses two more icons to complete this picture:


Interest payments are based on a rate represented by a circle called a converter.  The interest rate is multiplied by the existing debt, adding to the stock of debt. This feedback connection is shown by the red arrows called connectors.  Students can easily learn how to input numbers and equations in order to model this situation, learning some important lessons before getting their own credit cards.

Most people benefit from the discipline and practice necessary to draw stock and flow maps.  Their simplicity and clarity of design provide another communication tool representing how a system operates.  We help students gain an appreciation for rates of change, feedback, and other elements of systems when we integrate stock and flow thinking in our curriculum.

If you use any of the activities in The Shape of Change, have a look at the section in the newer editions of the text or on the web to see stock and flow explanations of each activity.

http://clexchange.org/cleproducts/shapeofchange_lessons.asp