Using Spectral Plots to Verify Expectations about Periodicity

Time series representing retail sales typically have an underlying annual periodicity, due to the usual peak in sales during the holiday season. Producing sales projections means building a model of the time series, which means identifying any periodic components. A plot of the time series may not always uncover the annual periodicity because time series contain random fluctuations that often mask the underlying structure.

Monthly sales data for a catalog company are stored in catalog.sav. See the topic Sample Files for more information. Before proceeding with sales projections, you want to confirm that the sales data exhibits an annual periodicity. A plot of the time series shows many peaks with an irregular spacing, so any underlying periodicity is not evident. Use the Spectral Plots procedure to identify any periodicity in the sales data.

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