question about using time in outcome eqns

[hebeil]

Hello,

I want to compute the probability of infection as a function of the prevalence of disease over the past 15 years (the prevalence being an outcome of my model). I'm assuming there's a way to do this in vensim... The timestep function from my understanding would give the prevalence from 15 years ago, is that correct, or is there a way to use timestep (or another function) to output a probability based on multiple time points of data?

Thanks for any help!!

Heather

[LAUJJL]

Hi

You must probably use an auxiliary variable defined as data to get exogenous variables defined for each time step.
Getting data si well explained in the user guide chapter 16.
Regards.
JJ

[bob@vensim.com]

I can't really figure out your question. If you have a model that generates prevalence over time for a communicable disease than it must have probability of infection based on contact as an input. If you are trying to find the fraction of the population that has, at one time or another, been infected, then keep a small demographic accounting with a stock of people never infected increased by births and decreased by deaths and a stock of people infected at some point increased by infections and decreased by deaths. If the disease does not affect mortality then you can use the same death rate (fractional deaths per person) on each stock. If it does affect mortality you will need to accomodate that. The birth and death rates can be read in exogenously as JJ suggests or you might be able to just use a constant over 15 years depending on the country you are looking at.