Additive Season, Linear Trend (More...)

To compute the smoothed values for the first season, initial values for the seasonal components are necessary. Also, to compute the smoothed value (forecast) for the first observation in the series, both estimates of S0 and T0 (initial trend) are necessary. By default, these values are computed as:
T0 = (Mk-M1)/((k-1)*p
where
k is the number of complete seasonal cycles
Mk is the mean for the last seasonal cycle
M1 is the mean for the first seasonal cycle
p is the length of the seasonal cycle
and S0 = M1 - T0/2