470 Statistical Methods
The line plot in Figure 11-26 demonstrates the seasonal nature of the data
and also allows you to observe individual values. Plots like this are some-
times called spaghetti plots, for obvious reasons.Applying the ACF to Seasonal Data
You can also use the autocorrelation function to display the seasonality of
the data. For a seasonal monthly series, the ACF should be very high at lag
12, because the current value should be strongly correlated with the value
from the same month in the previous year.To calculate the ACF:1 Return to the Liquor Sales worksheet.
2 Click Time Series from the StatPlus menu and then click ACF Plot.
3 Select Sales for the Data Values variable.
4 Click the up spin arrow to calculate the ACF up through a lag of 24.
5 Send the output to a new worksheet named Liquor Sales ACF.
6 Click OK.
Figure 11-27 shows the ACF of the liquor sales data.Figure 11-27
ACF of the
liquor sales
data