ARMA process predicts the future taking into account past values and errors. When making predictions, we often reach the past to find a pattern which can repeat itself in the future. Such patterns may have roots in seasons, days (business days and weekends), or time of day (day and night). However, rarely does same pattern happen multiple times. Unexpected events related to politics, the economy and daily life in general, disrupt any ready-to-use templates. Therefore, we need models like ARMA that simultaneously use past data as a template for estimates and can also include unpredictable events distorting this template.
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