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A Seasonal Index for Business
Author(s) -
Ittig Peter T.
Publication year - 1997
Publication title -
decision sciences
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.238
H-Index - 108
eISSN - 1540-5915
pISSN - 0011-7315
DOI - 10.1111/j.1540-5915.1997.tb01314.x
Subject(s) - index (typography) , multiplicative function , econometrics , computer science , set (abstract data type) , seasonality , statistics , mathematics , mathematical analysis , world wide web , programming language
This paper explores the problem of obtaining a multiplicative seasonal index for forecasting sales from a small set of historical data (as is common in business applications) and in the presence of a trend. It is shown that the standard method for generating a seasonal index (from a centered moving average) contains a systematic error. This error is transmitted through to forecasts that use the seasonal index and causes higher than necessary safety stocks and other consequences. The paper presents two alternative consistent methods for estimating the seasonal index in the presence of a trend, one for a multiplicative (nonlinear) trend and one for an additive (linear) trend. These methods may be run easily on a spreadsheet program or on statistical software. The nonlinear method is suggested as a convenient alternative to the standard method in many circumstances.