Before you continue with the example, it is worthwhile to consider how the system determined the values for the Time ID and Interval fields in the Data Set Selection window.

The Forecasting System requires that the input data set contain time series observations, with one observation for each time period. The observations must be sorted in increasing time order, and there must be no gaps in the sequence of observations. The time period of each observation must be identified by an ID variable, which is shown in the Time ID field.

If the data set contains a variable named DATE, TIME, or DATETIME, the system assumes that this variable is the SAS date or datetime valued ID variable, and the Time ID field is filled in automatically. The time ID variable for the SASHELP.CITIQTR data set is named DATE, and therefore the system set the Time ID field to DATE.

If the time ID variable for a data set is not named DATE, TIME, or DATETIME, you must specify the time ID variable name. You can specify the time ID variable either by typing the ID variable name in the Time ID field or by clicking the Select button.

If your data set does not contain a time ID variable with SAS date values, you can add a time ID variable using one of the windows described in Chapter 38, Creating Time ID Variables.

Once the time ID variable is known, the Forecasting System examines the ID values to determine the time interval between observations. The data set SASHELP.CITIQTR contains quarterly observations. Therefore, the system determined that the data have a quarterly interval, and set the Interval field to QTR.

If the system cannot determine the data frequency from the values of the time ID variable, you must specify the time interval between observations. You can specify the time interval by using the Interval combo box. In addition to the interval names provided in the pop-up list, you can type in more complex interval names to specify an interval that is a multiple of other intervals or that has date values in the middle of the interval (such as monthly data with time ID values falling on the 10th day of the month).

See Chapter 3, Working with Time Series Data, and Chapter 4, Date Intervals, Formats, and Functions, for more information about time intervals, SAS date values, and ID variables for time series data sets.