help in modeling demand in disasters
Posted: Sun Jun 03, 2018 5:41 am
Hi,
I want to model disaster demand and I have the following description for modeling it using Vensim:
The easiest way to model the disaster-generated demand is as an instantaneous constant G at the beginning (time step n=0). So you would have D(0)=G.
It is also often realistic to use a disaster-generated demand function G(n) to model the fact that a disaster may continue to generate additional demand after the initial time step. Two common expressions for this function are:
exponential decay G(n) = G_0*exp(-k*n), and
uptake-decay G(n) = G_0*n*exp(-k*n),
where G_0 and k are parameters that can be calibrated to model the dynamics of the disaster-generated demand. In this case, this function must be added to the outstanding demand function at each time step, so D(n) = D(n-1) +G(n).
It is also possible to add a “residual demand” term (G_r) to any of the formulas above to model the reality that the disaster-generated demand often never really goes away.
I have tried to model this using vensim. I modeled this formulation:
"G(n)"= G 0*n*exp(-k*n*Time)+"(G r)"
Using the above formulation I get a negative exponential graph, which is different from what I need to get. I attached the model together with the result that I want to get. I am not sure which part of my model is wrong? anyone can help please?
Kind regards,
I want to model disaster demand and I have the following description for modeling it using Vensim:
The easiest way to model the disaster-generated demand is as an instantaneous constant G at the beginning (time step n=0). So you would have D(0)=G.
It is also often realistic to use a disaster-generated demand function G(n) to model the fact that a disaster may continue to generate additional demand after the initial time step. Two common expressions for this function are:
exponential decay G(n) = G_0*exp(-k*n), and
uptake-decay G(n) = G_0*n*exp(-k*n),
where G_0 and k are parameters that can be calibrated to model the dynamics of the disaster-generated demand. In this case, this function must be added to the outstanding demand function at each time step, so D(n) = D(n-1) +G(n).
It is also possible to add a “residual demand” term (G_r) to any of the formulas above to model the reality that the disaster-generated demand often never really goes away.
I have tried to model this using vensim. I modeled this formulation:
"G(n)"= G 0*n*exp(-k*n*Time)+"(G r)"
Using the above formulation I get a negative exponential graph, which is different from what I need to get. I attached the model together with the result that I want to get. I am not sure which part of my model is wrong? anyone can help please?
Kind regards,