Ventana software support forum

Hi,

I try to use vensim to re-build the model of Future Electronics which was in dynamo under the textbook by Michael goodman Study notes in SD.

But, I face several problems.

a. 2 units errors discovered - I cannot figure out what is wrong as I follow the unit mentioned in the text.
b. the effective Testers become negative - seems something missing. In the text , it does not mention any initial stock qty about the testers.
c. I cannot understand why hiring rate (HR.KL) would have the initial seting as the text mentioned below

HR.KL=(FHM.K)(TT.K+T,K) 1,R
HR=20/69 1.1,N

HR - HIRING RATE (MEN/MONTH)
FHM - FRACTION HIRED PER MONTH (FRACTION/MONTH)
TT - TESTERS IN TRAINING (MEN)
T - TESTERS (MEN)

it is very enjoying and rewarding tasks to rebuild the classic dynamo models in vensim. hope i can get advise to help.

Thank you

Replicating old models is great!

a.

The unit error is due to lack of normalization in the table function. The equation is:

"Fraction Hired Per Month (FHM)"=
"Table of Hiring vs Complaints (THVC)"("Observed Complaints (OC)")
Units: 1/Month

It would be better to normalize the inputs and outputs, so the table itself was dimensionless. For example:

"Fraction Hired Per Month (FHM)"= Normal Hiring Fraction *
"Table of Hiring vs Complaints (THVC)"("Observed Complaints (OC)"/Normal Complaints)

b.

There's apparently a robustness issue in the decision rules. However, this is worsened by an inconsistency in the initialization that may be due to the translation. You have:

"Tester completing Training (TCT)"=
delay3("Hiring Rate (HR)","Training Delay (TD)")

"Testers in Training (TT)"= INTEG (
"Hiring Rate (HR)"-"Tester completing Training (TCT)",
20)

The 20 testers in training (initially) are not going to flow out, because the delay3 initialization does not recognize them. I think you want to use a delay3i with initial value of TT/TD.

c.

The initialization of the hiring rate is like a Vensim ACTIVE INITIAL. I think it's there to solve the problem I described above in b. 20/69 is a strange value though - the 20 is probably the initial TT, but not sure where the 69 comes from.