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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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Overview<br />

Options<br />

PPLOT 1225<br />

PPLOT (alias NPPLOT) produces probability plots of one or more sequence or time series<br />

variables. The variables can be standardized, differenced, and/or transformed before plotting.<br />

Expected normal values or deviations from expected normal values can be plotted.<br />

Modifying the Variables. You can request a natural log transformation of the sequence or<br />

time series variables using the LN subcommand and seasonal and nonseasonal differencing<br />

to any degree using the SDIFF and DIFF subcommands. With seasonal differencing, you<br />

can specify the periodicity on the PERIOD subcommand. You can also plot standardized<br />

series using the STANDARDIZE subcommand.<br />

Plot Type. You can request p-p (proportion-proportion) or q-q (quantile-quantile) plots on<br />

the TYPE subcommand. With the PLOT subcommand you can display normal plots,<br />

detrended plots, or both.<br />

Distribution Type. You can specify the distribution type on the DISTRIBUTION subcommand.<br />

The cumulative distribution function (CDF) and the inverse distribution function (IDF)<br />

for the specified distribution type are used to compute the expected values in the p-p and<br />

q-q plots, respectively.<br />

Score Calculations. On the FRACTION subcommand, you can specify one of several fractional<br />

rank formulas to use for estimating the empirical distribution in p-p plots and<br />

computing expected quantiles in q-q plots. You can specify the treatment of tied values on<br />

the TIE subcommand.<br />

Basic Specification<br />

The basic specification is one or more variable names.<br />

• For each variable specified, PPLOT produces two q-q plots of the observed values, one<br />

versus expected normal values and the other versus deviations from normal values. By<br />

default, expected normal values are calculated using Blom’s transformation.<br />

• Observed values define the horizontal axis, and expected normal values or deviations<br />

define the vertical axis.<br />

Subcommand Order<br />

• Subcommands can be specified in any order.<br />

<strong>Syntax</strong> Rules<br />

• VARIABLES can be specified only once.<br />

• Other subcommands can be specified more than once, but only the last specification of<br />

each one is executed.

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