SPSS® 12.0 Command Syntax Reference

312 CREATE
• Two lead orders indicate a range. For example, 1,5 indicates lead orders one through five.
A new series is created for each lead order in the range.
• The number of new series must equal the number of existing series specified times the
number of lead orders in the range.
• The last n cases at the end of the new series, where n equals the lead order, are assigned
the system-missing value.
• Missing values in the existing series are moved back and are assigned the system-missing
value in the new series.
Example
CREATE LEAD1 TO LEAD4 = LEAD(VARA,1,4).
• Four new series are created based on leads of VARA. LEAD1 is VARA led one step, LEAD2
is VARA led two steps, LEAD3 is VARA led three steps, and LEAD4 is VARA led four steps.
MA Function
MA produces new series based on the centered moving averages of existing series.
• The specification on MA is the name or names of the existing series and the span to be
used in averaging, in parentheses.
• A span must be specified; there is no default.
• If the specified span is odd, the MA is naturally associated with the middle term. If the
specified span is even, the MA is centered by averaging each pair of uncentered means
(Velleman and Hoaglin, 1981).
• After the initial span, a second span can be specified to indicate the minimum number of
values to use in averaging when the number specified for the initial span is unavailable.
This makes it possible to produce nonmissing values at or near the ends of the new series.
• The second span must be greater than or equal to 1 and less than or equal to the first span.
• The second span should be even (or 1) if the first span is even; it should be odd if the first
span is odd. Otherwise, the next higher span value will be used.
• If no second span is specified, the minimum span is simply the value of the first span.
• If the number of values specified for the span or the minimum span is not available, the
case in the new series is set to system-missing. Thus, unless a minimum span of 1 is specified,
the endpoints of the new series will contain system-missing values.
• When MA encounters an imbedded missing value in the existing series, it creates two subsets,
one containing cases before the missing value and one containing cases after the
missing value. Each subset is treated as a separate series for computational purposes.
• The endpoints of these subset series will have missing values according to the rules described
above for the endpoints of the entire series. Thus, if the minimum span is 1, the
endpoints of the subsets will be nonmissing; the only cases that will be missing in the new
series are cases that were missing in the original series.