Only Keep The Dates Beginning In January 2005 So Delete The Earlier Observations
Only keep the dates beginning in January 2005, so delete the earlier observations, and use the data through May 2018. Keep only the US data, both the seasonally unadjusted monthly (column B) and the seasonally adjusted annual (column G). Make a new column of seasonally adjusted monthly by dividing the annual data by 12. Make a column called “t” where t will go from 1 (Jan. 2005) to 161 (May 2018); make a t^2 column too (since, if you look at the data, you can see sales are slightly U-shaped; hence the quadratic). Also make a column “D” that is a dummy variable equal to one during the spring and summer months of March through August.
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