Ventana software support forum

Posted by ""Louis Macovsky"" <dynbiosys@verizon.net>
Greetings,

How do you in your practice of causal loop diagramming with clients convey the concept that i.e. Loop Y is not activated until some threshhold is reached in Loop X? Without some accompanying narrative, the client may be mislead. Is there a way to depict this in the CLD diagram without a narrative?

Is it your practice to have a representative label of the presence of a threshhold placed in the CLD? If so, how do you represent polarity (when the threshhold is static)? Is it your practice to use a labeled threshhold outside the loop; if so, how do you explain the polarity?

Example: In toxicology and pharmacology, many use the concept of a NOEL (No Observable Effect Level). At some point the level of exposure, dose, or tissue concentration will cause a LOEL (Lowest Observed Effect Level) and a level of ""effect"" is activated in the CLD. Representation of this in the dynamic model is relatively basic but how about the CLD? Thank you, Lou

Louis Macovsky
Dynamic BioSystems
Wilsonville, OR, USA
Posted by ""Louis Macovsky"" <dynbiosys@verizon.net>
posting date Fri, 21 Oct 2005 08:22:37 -0700

Posted by George Richardson <gpr@albany.edu>
On Oct 22, 2005, at 7:09 AM, Louis Macovsky dynbiosys verizon.net wrote:


>> How do you in your practice of causal loop diagramming with clients
>> convey the concept that i.e. Loop Y is not activated until some
>> threshhold is reached in Loop X? Without some accompanying
>> narrative, the client may be mislead. Is there a way to depict this
>> in the CLD diagram without a narrative?


Common practice is to leave out the threshold level (or reference value, or ""normal value"") in more qualitative maps and to show it explicitly in more formal maps. Whether to show it or not depends, I would say, on your audience for the diagram, the complexity or detail they're interested in, and the clarity of the resulting diagram.

>>
>> Is it your practice to have a representative label of the presence of
>> a threshhold placed in the CLD? If so, how do you represent polarity
>> (when the threshhold is static)? Is it your practice to use a
>> labeled threshhold outside the loop; if so, how do you explain the
>> polarity?
>>
>> Example: In toxicology and pharmacology, many use the concept of a
>> NOEL (No Observable Effect Level). At some point the level of
>> exposure, dose, or tissue concentration will cause a LOEL (Lowest
>> Observed Effect Level) and a level of ""effect"" is activated in the
>> CLD. Representation of this in the dynamic model is relatively basic
>> but how about the CLD?


I'd say Tissue Concentration (or whatever) and LOEL would always enter into your map (if you decide to include LOEL) with *opposite
signs*. E.g., if a higher Tissue Concentration relative to LOEL
would cause something else (say Perceived Effect of Concentration) to increase, then the arrow from Tissue Concentration to Perceived Effect would be positive and the arrow from LOEL would be negative. The interpretation of the polarity on the arrow from LOEL is that the higher the lowest observable effect level, then the *lower* the Perceived Effect of Concentration. You should always be able to tell a sensible sounding story from having the polarities of a ""thing"" and its threshold level to be opposite.

The question you raise is important. I think it is true that all ""effects"" that we would try to capture in maps or models are always best thought of as ""relative."" It's not the effect of the absolute level of the concentration we observe or think about; it's the effect of the *relative* level of the concentration -- relative to some base rate, or reference condition, threshold, or ""normal.""

Examples abound. E.g., it's not the level of the Federal Debt (or my credit card debt) that we worry about -- it's the level of debt relative to something, e.g., Federal Debt compared to GDP. For companies or people, we might say something like a ""debt-to-equity ratio,"" but I'd even say it's the current debt-to-equity ratio *compared to* (relative to) some reference or threshold debt-to- equity ratio. It's a ""debt-to-equity ratio ratio,"" DER/DER normal.

You can see the modeling pattern in our literature going back a long way. Forrester formulated the effects of Food per Capita, or Pollution, or whatever in World Dynamics as ratios to reference conditions -- the Food Ratio in World2 is not just ""food per capita,"" but rather Food per Capita divided by Normal (1970) Food per Capita.

You can also see it in our current thoughts about gasoline prices. $3 a gallon is perceived to be high, but clearly it is high relative a standard we have gotten used to in the U.S. The absolute number can't be what's causally important, since Europe would respond completely differently to gas at $3 a gallon!

And you can see it in Vensim's warning if you use a ""dimensioned argument"" in a graphical lookup function. It's Vensim's way of trying to urge us to model pressures and their effects as relative to something, usually expressed as dimensionless ratios.

So the really good question here is how one balances the need in less formal maps for simplicity and clarity with the real need to keep the notion of *relative* values in our thinking about forces in social and (I think) natural systems. The reference value has to be there in our minds, whether we show it or not in the map.

..George

George P. Richardson
Chair of public administration and policy
Rockefeller College of Public Affairs and Policy
University at Albany - SUNY, Albany, NY 12222
gpr@albany.edu *http://www.albany.edu/~gpr
Posted by George Richardson <gpr@albany.edu>
posting date Sun, 23 Oct 2005 12:05:19 -0400

Posted by ""John Gunkler"" <jgunkler@sprintmail.com>
This is quite simplistic, but one way to deal with the issue of keeping the picture simpler is simply to call such items (that actually enter into the dynamics as ratios) by a name that implies something else is going on but doesn't create confusion -- something like ""relative debt-to-equity ratio"" or ""relative price per gallon.""

The effect of this on the audience, I believe, is in the first instance that they simply interpret that factor without the word ""relative."" (So, in George's example, they simply think ""price per gallon."") But, at the same time, they're aware there's something else going on -- something they don't need to pay attention to immediately, but might want to learn more about later.

You might want to test this out with some people and see if I'm correct.


John Gunkler
Posted by ""John Gunkler"" <jgunkler@sprintmail.com>
posting date Wed, 26 Oct 2005 22:11:48 -0500