3.9 A company’s six-monthly sales figures for a five-year period are given below (in millions of dollars). 1992 1993 1994 1995 1996 Jan–June 1.09 1.10 1.08 1.04 1.03 ...

3.9
A company’s six-monthly sales figures for a five-year period are given below (in millions of dollars).

(a)
Obtain a trend estimate using a centered 2 MA smoother and compute the de-trended figures assuming an additive decomposition.

(b)
Assuming the seasonality is not changing over time, cal- culate seasonally adjusted figures for 1996.

(c)
Suppose several more years of data were available. Ex- plain in about two sentences how the classical decom- position method could be modified to allow for seasonal effects changing over time.

Australian civilian labor force (’000 persons)

1980 1985 1990 1995

Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

Figure 3-13:
STL decomposition of the number of persons in the civilian labor force in Australia each month from February 1978 to August 1995.