On the importance of verifying forecasting results

We discuss the various sources of error in numerical computations with the use of examples from the literature relevant to time series analysis. We also submit a case where, by manual verification, we were able to discover a plausible forecast to be erroneous due to a number of software flaws in the XLSTAT addin for Microsoft Excel. Furthermore, after discussing the alternative techniques for implementing the ARIMA (AutoRegressive Integrated Moving Average) methodology on a computer, we show that different approaches can cause considerable discrepancies in the results across different programs, and even within a single software system.