I have four variables and would like to construct a VAR model I would then like to make a VAR forecast on one of the variables using my own data for the forecasts of the other three variables. Is there an R package or process from the model fit that allows me to do this?
closed as off topic by gung, Macro, Andy W, Peter Ellis, csgillespie Dec 1 '12 at 15:51
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R packages to do VAR modelling are listed in the Time Series View.
To see the impact of shocks to particular series, you'd normally compute impulse-response functions. All VAR implementations should do for you, e.g. in vars.
However, explicitly fixing the path of one series is a bit different, and perhaps also a bit odd because by doing so you are treating that series as exogenous, whereas one normally fits a VAR because the series are considered to be endogenous. Normally one applies VARX models for the first sort of assumption, but I think that's probably not what you have in mind for scenario planning.
That said, you could probably treat the problem as an sequence of one step prediction problems in AR rather than MA formulation with a fitted VAR. You'd have to write a function to sample a new step forward for all other series at each step, conditional on where you wanted the controlled one to be. If you do that enough times and summarise it you'll get a conditional forecast.
So, the answer to your question is: no, I don't think so. And the reason there isn't (so far as I know) is given above.