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I started working with ARIMA and ETS and I know that we can either specify components ourselves or leave it for the model (auto). My question is - should we try to pick the components ourselves or leave them on auto and let the model do the work?
If ARIMA asks for d - do we need to provide data after differencing or does model do differencing itself? If we need to do it - do we start with seasonal or nonseasonal differencing?
If you would like to be as thorough as possible, you can customize the auto model creation parameters. However, this can significantly increase the run-time of your workflow (especially if you select the full enumeration option):
With regards to ETS tool, if you can identify whether or not your time series data represents an additive model or a multiplicative model, you could specify. If not, you will want to allow the auto options to specify the model.
In my experience, the auto options for both the ARIMA tool and the ETS tool do a reasonable job of identifying the correct functional form of the model. Also, I’ve found that Forecasting: Principles and Practice provides a very approachable overview of these modeling types, so it may be worth reviewing this text.