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SPSS® 12.0 Command Syntax Reference

SPSS® 12.0 Command Syntax Reference

SPSS® 12.0 Command Syntax Reference

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174 CASEPLOT<br />

DIFF Subcommand<br />

DIFF specifies the degree of differencing used to convert a nonstationary variable to a stationary<br />

one with a constant mean and variance before plotting.<br />

• You can specify any positive integer on DIFF.<br />

• If DIFF is specified without a value, the default is 1.<br />

• The number of values displayed decreases by 1 for each degree of differencing.<br />

Example<br />

CASEPLOT TICKETS<br />

/DIFF=2.<br />

• In this example, TICKETS is differenced twice before plotting.<br />

SDIFF Subcommand<br />

If the variable exhibits a seasonal or periodic pattern, you can use the SDIFF subcommand to<br />

seasonally difference a variable before plotting.<br />

• The specification on SDIFF indicates the degree of seasonal differencing and can be any<br />

positive integer.<br />

• If SDIFF is specified without a value, the degree of seasonal differencing defaults to 1.<br />

• The number of seasons displayed decreases by 1 for each degree of seasonal differencing.<br />

• The length of the period used by SDIFF is specified on the PERIOD subcommand. If the<br />

PERIOD subcommand is not specified, the periodicity established on the TSET or DATE<br />

command is used (see the PERIOD subcommand below).<br />

PERIOD Subcommand<br />

PERIOD indicates the length of the period to be used by the SDIFF subcommand.<br />

• The specification on PERIOD indicates how many observations are in one period or season<br />

and can be any positive integer.<br />

• PERIOD is ignored if it is used without the SDIFF subcommand.<br />

• If PERIOD is not specified, the periodicity established on TSET PERIOD is in effect. If<br />

TSET PERIOD is not specified either, the periodicity established on the DATE command<br />

is used. If periodicity is not established anywhere, the SDIFF subcommand will not be<br />

executed.<br />

Example<br />

CASEPLOT TICKETS<br />

/SDIFF=1<br />

/PERIOD=12.<br />

• This command applies one degree of seasonal differencing with 12 observations per season<br />

to TICKETS before plotting.

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