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Connections between models and reality
Posted by Steve Kipp on 3/9/2004
In Reply To:Connections between models and reality Posted by Mohamad Morovati on 3/6/2004
This has been an interesting discussion. I'm not sure there are hard and fast technical rules for stocks and flows aside from conceptual and unit consistency (you can't have a flow of "pollution" into the stock "water in the lake"), although many good rules of thumb have been shared here (nouns vs verbs; think about what happens when you take a picture). Mohamad, I’m not sure I agree that you can’t directly measure flows (what about wind speed, electrical or liquid currents, births or deaths per year, kilometers per hour in a car, and so on?), but I agree that there are many stocks whose flows have no common names (we name the inflow to a bank account “deposits”, but have no name in English for the inflow into the stock of “anger”, so I usually call it “increasing anger”). It would be interesting to compare how different cultures have given names to different flows!
I am concerned, though; if I were a Systems newbie, I think I would be a bit spooked by this discussion, we seem to be “all over the place” here. My bottom line for any stock/flow or computer simulation exercise is driven by questions of effective communication. Who are my participants? What are we trying to communicate/explore together? Do I have a mental model about a system that I want to illustrate, then explore? How will stocks and flows or computer simulation increase clarity? This helps to make decisions about the level of technical detail we wish to share. Less eye-glazing results! The level of appropriate detail is usually WAY less than I initially assume...go for “the simplicity on the other side of complexity”. Apply Occams Razor.
With k-12 students, I pretty much insist on them having an initial (VERY simple) bathtub or bank account model experience (with an animated stock and a graphical output visible simultaneously so they make the connection). Then we can have a pretty good discussion just with stock/flow mapping on the board, and perhaps do some more simulation.
Some of the most powerful examples I have seen mentioned in this thread are co2, population, debt/deficit, and AIDS. I think they are powerful applications of stocks and flows because 1)they are important 2)confusion about them reigns in society at large, and 3)just the distinction between stock and flow can help to increase the clarity of the discussion. For example, if the rate of co2 emissions goes down, the stock of co2 continues to go up, but at a slower rate...we tend to forget that most people simply don’t understand this. When the government brags about a reduction in the federal deficit, very few folks understand that the debt still goes up unless payments (outflow on the debt) were greater than the deficit (inflow) that year (not one out of 25 middle school teachers in a class I presented to recently knew this distinction!). World population “growth rates” are declining, but population is still climbing because “births per thousand” are still higher than “deaths per thousand” overall. And with AIDS in the U.S., new infection rates are dropping, but the AIDS death rate has declined due to new treatments, so the population of U.S. AIDS patients is increasing dramatically, leading to interesting problems like Lazarus syndrome. This is not true in other countries, where infection percentages can be over 40% of the population and there are currently very few treatment options available, and death rates are soaring.
And then there are the “soft variable” systems, like anger, or critical thinking. It is my mental model that we shouldn’t avoid these just because they can’t be easily measured. You can have a great stock/flow discussion with kids 10 years old that there are two ways to outflow anger through action: socially appropriate actions, which outflow anger with few negative side effects; and socially inappropriate actions, which, although they do successfully outflow anger in the short term, tend to feed back through the system to increase anger when punishments occur later in time! Then you can talk about specific examples of appropriate and inappropriate anger-decreasing actions, a very good useful discussion to have.
Steve
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