Trend Analysis with Seasonality

The following data is retail sales for the ABC Company.

Period

1997

1998

1999

Jan.

98.8

105.6

106.4

Feb.

95.6

99.7

105.8

Mar.

110.2

114.2

120.4

Apr.

113.1

115.7

125.4

May

120.3

125.4

129.1

Jun.

115.0

120.4

129.0

Jul.

115.5

120.7

129.3

Aug.

121.1

124.1

131.5

Sept.

113.8

124.4

124.5

Oct.

115.8

123.8

128.3

Nov.

118.1

121.4

126.9

Dec.

138.6

152.1

157.2

a. Compute a regression line for the data to predict sales by month for the year 2000.

b. Calculate monthly seasonal indices for each month.

c. Compute a seasonally adjusted forecast for each month in 2000.

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