Major Loops vs. Minor Loops

[Eliot Rich]

What's the difference between a major loop and a minor loop?

I've seen the term used in a formal context in discussions of control theory and loop dominance (ref: Mojtahedzadeh, http://onlinelibrary.wiley.com/doi/10.1002/sdr.399/pdf, p.456), and in a more descriptive way (ref: Forrester', "Market growth as influenced by capital investment", D-4079-1, pg 10, http://www.clexchange.org/ftp/documents ... growth.pdf).

Is there a solid and accessible heuristic that distingushes major from minor? Does it link back to the loop's influence on dominance?

Thanks.

Eliot

[Luis Luna-Reyes]

It has to do with the number of stocks in the loop. Major loops have two or more stocks. Minor loops have only one stock.

[Eliot Rich]

Hello Luis.

Might I ask you to expand your response a bit?

Thanks.

Eliot

[Robert Eberlein]

Luis's response is concise an complete. A minor feedback loop involves only a single level, a major two or more levels. The distinction is important because a minor negative can only converge (never oscillate) and a minor positive loop can only grow at an ever increasing rate. Once you add another level, things change. Thus the major loop Mohammad refers to involving inventory and workforce (two levels).

Of course, one always has to be on the lookout for secondary loops, even within a minor feedback loop. One of my favorite pedagogical examples is the sales force, sales, revenue, budget to sales, sales force positive loop. You can formulate this with one level (sales force) and still get collapse if the price is too high, the sales people too ineffective, or their budget allocation too small. The reason this is true is that the workforce adjustment process is itself a negative loop. Thus, when the two combine you don't necessarily get growth.

One other thing to note is that if you use difference equations, a minor negative loop can display expanding oscillations. This can manifest in models when you have adjustment times smaller that the computational interval and use Euler integration and it can be confusing.

Not nearly as concise as Luis - but I hope that is helpful.

[Eliot Rich]

Bob,

Thanks for the details. One more question from the floor - the source of the terms "Major" or "Minor"?

Best,

Eliot

[Robert Eberlein]

Hi Eliot,

I was thinking about that when I replied. I have always assumed the terminology simply comes from involving only one as minor relative to involving more than one, but I really don't know for sure. I was tempted to say something about harmonics and keys, but that would be pure fabrication. If anyone knows the etymology of the terms I would also be curious.