Thus the current sales rate turns out to be a weighted
average of the sales rate of the last period and the
current sales seasonally adjusted by the index of a
year ago. Numerical values of S¯ t from Eq. (4) can be
substituted in Eq. (2) to determine Pt. However,
substituting Eq. (4) in Eq. (2) gives us an explicit
analytic expression for the new seasonal ratio: