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My data is strictly positive (and zero values), and the output of a forecast should be only positive (and zero values) too.
My equation (simplified) looks something like this: y = 3 * x - 1 * z
So it's possible that a negative value arises if e.g. x is 0 and z is above 0.
But i don't want negative values, can i somehow specify that negative forecasts of y should be converted to 0?

Perform the forecast, then recode it: where yf is the name of your forecast series.

Thanks.

But i would also need the forecast evaluation under the condition that all negative values are zeros. Is it possible to somehow specify it in the estimation equation?

No. But you can always calculate the forecast evaluations yourself using the @mape, @rmse, @theil functions.

Thanks i'll look it up.