QUERY World dynamics model constant values (SD6660)

[SDMAIL Martin Schaffernicht]

Posted by  Martin Schaffernicht <martin at utalca.cl>

Hi,

at Boston I bought the "World dynamics" book and back at home, I gave 
it to some students - I want them to build a spanish language learning 
environment based on the model exposed in the book.  In some of the 
equations, a "clip" funtion is used.  I found out that this was part of 
DYNAMO and it alternatively returns one of two values (or variables), 
depending on a comparison.

So it can be replaced by a convenient IF THEN ELSE; that is how it has 
been done in the Vensim replica of "world dynamics" on Tom Fiddaman's 
website.

However, in the "world dynamics" model, several times the two values amongst 
which the funtion shall select have the same value.  For instance:

births = Population * IF THEN ELSE ( Time > switch time 1, birth rate normal 1, 
     birth rate normal )* births material multiplier* births crowding multiplier* 
     births food multiplier* births pollution multiplier
birth rate normal  = 0.04
birth rate normal 1  = 0.04

There seems to be no reason to say "IF THEN ELSE ( Time > switch time 1, 
birth rate normal 1, birth rate normal )", when "birth rate normal 1" = 
"birth rate normal".  I guess Jay Forrester did have a reason to use the function, 
but in the book chapters surrounding the equation, I connot find it.

What was the reason?  Can someone please tell me?

Thanks,

Martin Schaffernicht
Posted by  Martin Schaffernicht <martin at utalca.cl>
posting date  Fri, 12 Oct 2007 15:21:12 -0400