Hi Christoph, it sounds like you didn't aggregate the data by quarter?
When using "R-Triple Exponential Smoothing" the input data must be aggregated by the period type you want to predict. So data that goes into the algorithm must contain only one record per quarter.
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However, maybe the "Triple Exponential Smoothing" algorithm fits better for your use. There is no "R-" at the beginning of the name. This means this algorithm is not using the open-source R but the algorithm was implemented by SAP. The configuration of the two is slightly different.
With "Triple Exponential Smoothing" you get the option "Consider date column", which sounds like what you are after. Here it uses the time hierachy of a date field to aggregate the data for you. So you can have multiple records per period. The algorithm then aggregates the data per period (ie quarter) and make the prediction for the next periods.
