The most common use of the X12 procedure is to produce a seasonally adjusted series. Eliminating the seasonal component from an economic series facilitates comparison among consecutive months or quarters. A plot of the seasonally adjusted series is often more informative about trends or location in a business cycle than a plot of the unadjusted series.
The following example shows how to use PROC X12 to produce a seasonally adjusted series, , from an original series .
In the multiplicative model, the trend cycle component keeps the same scale as the original series , while , , and vary around 1.0. In all displayed tables, these latter components are expressed as percentages and thus vary around 100.0 (in the additive case, they vary around 0.0). However, in the output data set, the data displayed as percentages in the displayed output are expressed as the decimal equivalent and thus vary around 1.0 in the multiplicative case.
The naming convention used in PROC X12 for the tables follows the convention used in the Census Bureau’s X-12-ARIMA program; see X-12-ARIMA Reference Manual (U.S. Bureau of the Census; 2009c), X-12-ARIMA Quick Reference for Windows (PC) (U.S. Bureau of the Census; 2009b), and X-12-ARIMA Quick Reference for UNIX/Linux (U.S. Bureau of the Census; 2009a). Also see the section Displayed Output, ODS Table Names, and OUTPUT Tablename Keywords. The table names are outlined in Table 37.12.
The tables that correspond to parts A through C are intermediate calculations. The final estimates of the individual components are found in the D tables: Table D10 contains the final seasonal factors, Table D12 contains the final trend cycle, and Table D13 contains the final irregular series. If you are primarily interested in seasonally adjusting a series without consideration of intermediate calculations or diagnostics, you need to look only at Table D11, the final seasonally adjusted series. Tables in part E contain information about extreme values and changes in the original and seasonally adjusted series. The tables in part F are seasonal adjustment quality measures. Spectral analysis is performed in part G. For further information about the tables produced by the X11 statement, see Ladiray and Quenneville (2001).