IRCHART Statement: SHEWHART Procedure

Output Data Sets

OUTLIMITS= Data Set

The OUTLIMITS= data set saves control limits and control limit parameters. The following variables can be saved:

Table 18.19: OUTLIMITS= Data Set

Variable

Description

_ALPHA_

Probability ($\alpha $) of exceeding limits

_CP_

Capability index $C_{p}$

_CPK_

Capability index $C_{pk}$

_CPL_

Capability index $CPL$

_CPM_

Capability index $C_{pm}$

_CPU_

Capability index $CPU$

_INDEX_

Optional identifier for the control limits specified with the OUTINDEX= option

_LCLI_

Lower control limit for individual measurements

_LCLR_

Lower control limit for moving ranges

_LIMITN_

Number of consecutive measurements used to compute moving ranges

_LSL_

Lower specification limit

_MEAN_

Process mean

_R_

Value of central line on moving range chart

_SIGMAS_

Multiple (k) of standard error of individual measurement or moving range

_STDDEV_

Process standard deviation ($\hat{\sigma }$ or $\sigma _{0}$)

_SUBGRP_

Subgroup-variable specified in the IRCHART statement

_TARGET_

Target value

_TYPE_

Type (estimate or standard value) of _MEAN_ and _STDDEV_

_UCLI_

Upper control limit for individual measurements

_UCLR_

Upper control limit for moving ranges range

_USL_

Upper specification limit

_VAR_

Process specified in the IRCHART statement


Notes:

  1. If the limits are defined in terms of a multiple k of the standard errors of $X_{i}$ and $R_{i}$, the value of _ALPHA_ is computed as $\alpha =2(1-\Phi (k))$, where $\Phi (\cdot )$ is the standard normal distribution function.

  2. If the limits are probability limits, the value of _SIGMAS_ is computed as $k=\Phi ^{-1}(1-\alpha /2)$, where $\Phi ^{-1}$ is the inverse standard normal distribution function.

  3. The variables _CP_, _CPK_, _CPL_, _CPU_, _LSL_, and _USL_ are included only if you provide specification limits with the LSL= and USL= options. The variables _CPM_ and _TARGET_ are included if, in addition, you provide a target value with the TARGET= option. See Capability Indices for computational details.

  4. Optional BY variables are saved in the OUTLIMITS= data set.

The OUTLIMITS= data set contains one observation for each process specified in the IRCHART statement. For an example, see Saving Control Limits.

OUTHISTORY= Data Set

The OUTHISTORY= data set saves individual measurements and moving ranges. The following variables are saved:

  • the subgroup-variable

  • an individual measurements variable named by process

  • a moving range variable named by process suffixed with R

Given a process name that contains 32 characters, the procedure first shortens the name to its first 16 characters and its last 15 characters, and then it adds the suffix.

A variable containing the moving ranges is created for each process specified in the IRCHART statement. For example, consider the following statements:

proc shewhart data=Steel;
   irchart (Width Diameter)*Lot / outhistory=Summary;
run;

The data set Summary contains variables named Lot, Width, WidthR, Diameter, and DiameterR.

Additionally, the following variables, if specified, are included:

  • BY variables

  • block-variables

  • symbol-variable

  • ID variables

  • _PHASE_ (if the OUTPHASE= option is specified)

For an example of an OUTHISTORY= data set, see Saving Individual Measurements and Moving Ranges.

OUTTABLE= Data Set

The OUTTABLE= data set saves individual measurements, moving ranges, control limits, and related information. Table 18.20 lists the variables that are saved.

Table 18.20: OUTTABLE= Data Set Variables

Variable

Description

_ALPHA_

Probability ($\alpha $) of exceeding control limits

_EXLIM_

Control limit exceeded on individual measurements chart

_EXLIMR_

Control limit exceeded on moving range chart

_LCLI_

Lower control limit for individual measurements

_LCLR_

Lower control limit for moving range

_LIMITN_

Number of consecutive measurements used to compute moving ranges

_MEAN_

Process mean

_R_

Average range

_SIGMAS_

Multiple (k) of the standard error associated with control limits

Subgroup

Values of the subgroup variable

_SUBI_

Individual measurement

_SUBR_

Moving range

_TESTS_

Tests for special causes signaled on individual measurements chart

_UCLI_

Upper control limit for individual measurements

_UCLR_

Upper control limit for moving range

_VAR_

Process specified in the IRCHART statement


In addition, the following variables, if specified, are included:

  • BY variables

  • block-variables

  • symbol-variable

  • ID variables

  • _PHASE_ (if the READPHASES= option is specified)

Notes:

  1. Either the variable _ALPHA_ or the variable _SIGMAS_ is saved, depending on how the control limits are defined (with the ALPHA= or SIGMAS= options, respectively, or with the corresponding variables in a LIMITS= data set).

  2. The variable _TESTS_ is saved if you specify the TESTS= option. The kth character of a value of _TESTS_ is k if Test k is positive at that subgroup. For example, if you request all eight tests and Tests 2 and 8 are positive for a given subgroup, the value of _TESTS_ has a 2 for the second character, an 8 for the eighth character, and blanks for the other six characters.

  3. The variables _EXLIM_, _EXLIMR_, and _TESTS_ are character variables of length 8. The variable _PHASE_ is a character variable of length 48. The variable _VAR_ is a character variable whose length is no greater than 32. All other variables are numeric.

For an example, see Saving Control Limits.