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Hello I am trying to forecast using different exponential smoothing methods(Linear and Winter's). For the optimal parameters, I am getting negative values of the forecasats. I am assuming it means that the values will be zero, since it is a sales forecast. I wanted to know if negative values denote something wrong with the model or is it possible to have negative values in forecast. Since it is just a model, I think we can get negative values for certain type of time series.

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Holt's or Winter-Holt's exponential smoothing methods can give negative values for purely non-negative input values because of the trend factor which acts as a kind of inertia, which can drive the time series below zero. Normal exponential smoothing doesn't have this problem, it's always smoothing inwards, it never overshoots.

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  • $\begingroup$ However, the prediction intervals can get negative. How can one prevent it? $\endgroup$ – Waldir Leoncio Aug 14 '15 at 19:59
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You can get negative values for certain kinds of models. You might want to explore more complicated models than simple exponential smoothing.

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