The process of breaking a problem into subproblems, solving them, and then combining the solutions to get an overall solution.  MacGregor (2001) provides evidence on the value of this procedure for judgmental forecasting. Typically, decomposition refers to multiplicative breakdowns, but sometimes it applies to additive breakdowns. Additive breakdowns, however, are usually called disaggregate forecasting or segmentation. Time series are often decomposed by level, trend, cycle, seasonality, and error.