SPSS® 12.0 Command Syntax Reference

314 CREATE
• 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 RMED 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.
Example
CREATE TICKRMED = RMED(TICKETS,4,2).
• This example creates the series TICKRMED using centered running median values of the
series TICKETS.
• A span of 4 is used for computing medians. At the endpoints, where four values are not
available, the median is based on the specified minimum of two values.
SDIFF Function
SDIFF produces new series based on seasonal differences of existing series.
• The specification on SDIFF is the name or names of the existing series, the degree of differencing,
and, optionally, the periodicity, all in parentheses.
• The degree of differencing must be specified; there is no default.
• Since the number of seasons used in the calculations decreases by 1 for each order of differencing,
system-missing values will appear at the beginning of the new series.
• You can specify only one degree of differencing per SDIFF function.
• If no periodicity is specified, the periodicity established on TSET PERIOD is in effect. If
TSET PERIOD has not been specified, the periodicity established on the DATE command is
used. If periodicity was not established anywhere, the SDIFF function cannot be executed.
• If either of the pair of values involved in a seasonal difference computation is missing,
the result is set to system-missing in the new series.
Example
CREATE SDVAR = SDIFF(VARA,1,12).
• The series SDVAR is created by applying one seasonal difference with a periodicity of 12
to the series VARA.