Handling a Delay Function

[jbh]

Attached is the first stage of a model we are working on. First of all, we are very new to buiding Sys Dyn. models.

In this model, we are having a difficult time managing the units. The one spot we are struggling with now is how to go from # of people in the "Personnel Gap" variable to people/month in the "Hire Rate" variable. We figured we could add another variable going into "Hire Rate" that would set the # of months, but we were trying to use a delay function.

Can you offer any guidance on how to do this or point us toward the documentation of how to do this?

Thanks,
jbh

[bob@vensim.com]

Whenever you have a goal and a current state you need to take action to adjust that state over time. The classic formulation is

hire rate = (target workforce - workforce)/time to adjust workforce

If you make a graph of workforce and target workforce you will see that workforce is a first order exponential delay of its target. If you want a hire order delay (usually not a good idea) you would use something like

hire rate = DELAY3(indicated hire rate,hiring delay)

indicated hire rate = (target workforce - workforce)/time to adjust workforce

I would recommend taking a look at the molecules http://www.vensim.com/molecule.html as there are a number of related formulations contained in these.