Overview (PPLOT command)

PPLOT (alias NPPLOT) produces probability plots of one or more sequence or time series variables. The variables can be standardized, differenced, and/or transformed before plotting. Expected normal values or deviations from expected normal values can be plotted.

Options

Variable Modification. You can use the LN subcommand to request a natural log transformation of the sequence or time series variables, and you can use the SDIFF and DIFF subcommands to request seasonal and nonseasonal differencing to any degree. With seasonal differencing, you can specify the periodicity on the PERIOD subcommand. You can also plot standardized series by using the STANDARDIZE subcommand.

Plot Type. You can request p-p (proportion-proportion) or q-q (quantile-quantile) plots on the TYPE subcommand. With the PLOT subcommand, you can display normal plots, detrended plots, or both.

Distribution Type. You can specify the distribution type on the DISTRIBUTION subcommand. The cumulative distribution function (CDF) and the inverse distribution function (IDF) for the specified distribution type are used to compute the expected values in the p-p and q-q plots, respectively.

Score Calculations. On the FRACTION subcommand, you can specify one of several fractional rank formulas to use for estimating the empirical distribution in p-p plots and computing expected quantiles in q-q plots. You can specify the treatment of tied values on the TIE subcommand.

Templates. You can specify a template, using the TEMPLATE subcommand, to override the default chart attribute settings on your system.

Basic Specification

The basic specification is one or more variable names.

  • For each specified variable, PPLOT produces two q-q plots of the observed values (one plot versus expected normal values and the other plot versus deviations from normal values. By default, expected normal values are calculated by using Blom’s transformation.
  • Observed values define the horizontal axis, and expected normal values or deviations define the vertical axis.

Subcommand Order

  • Subcommands can be specified in any order.

Syntax Rules

  • VARIABLES can be specified only once.
  • Other subcommands can be specified more than once, but only the last specification of each subcommand is executed.

Operations

  • Subcommand specifications apply to all plots that are produced by PPLOT.
  • If the LN subcommand is specified, any differencing or standardization that is requested on that PPLOT is done on the log-transformed series.
  • If differencing (DIFF or SDIFF) is specified, any standardization is done on the differenced series.

Limitations

  • A maximum of 1 VARIABLES subcommand is allowed. There is no limit on the number of variables that are named on the list.