Observations in the data set analyzed by the SPECTRA procedure should form ordered, equally spaced time series. No more than 99 variables can be included in the analysis.

Data are often detrended before analysis by the SPECTRA procedure. This can be done by using the residuals output by a SAS regression procedure. Optionally, the data can be centered using the ADJMEAN option in the PROC SPECTRA statement, since the zero periodogram ordinate corresponding to the mean is of little interest from the point of view of spectral analysis.