Ventana software support forum

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

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

Posted by Jay Forrester <jforestr@MIT.EDU>

The switches allow one to change the values of parameters at a specified
time for testing alternative policies. See simulation runs in Figures
5-1 through 6-8.

[Host's note: The figures referred to are in the book
""World Dynamics,"" 1971, 1973 by Jay Forrester which is available from
http://www.pegasuscom.com if you can't find a copy in the library ]

Posted by Jay Forrester <jforestr@MIT.EDU>
posting date Sat, 13 Oct 2007 15:12:59 -0400
_______________________________________________

Posted by Jay Forrester <jforestr@MIT.EDU>

The switches allow one to change the values of parameters at a specified
time for testing alternative policies. See simulation runs in Figures
5-1 through 6-8.

[Host's note: The figures referred to are in the book
""World Dynamics,"" 1971, 1973 by Jay Forrester which is available from
http://www.pegasuscom.com if you can't find a copy in the library ]

Posted by Jay Forrester <jforestr@MIT.EDU>
posting date Sat, 13 Oct 2007 15:12:59 -0400
_______________________________________________