The Correction of Chronologic Series’ Seasonal Fluctuations according to Seasonal Simultaneous Additive and Multiplicative Effects

In this study we set the problem of the probable existence of an additive and multiplicative mixed seasonality. In this context, we show by some simulation that the seasonality correction according to a pure additive or a pure multiplicative scheme leads to biased estimators of the coefficients and, consequently, in the calculation of seasonally adjusted series which is necessary for quantitative demand analysis. The use of an analytical resolution technique allowing to estimate simultaneously the trend coefficients and the additive and multiplicative seasonal coefficients. perfectly works if the series is affected by a simple linear trend .In this case, the estimation gives the theoretical seasonal coefficients. An application study to the mobile phone market, new product split in two markets : professional and individuals allows to evaluate the contribution of the methodology.