XSCHART Statement: SHEWHART Procedure

Output Data Sets

OUTLIMITS= Data Set

The OUTLIMITS= data set saves control limits and control limit parameters. Table 18.66 lists the variables that are saved.

Table 18.66: 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
`_LCLS_`	Lower control limit for subgroup standard deviation
`_LCLX_`	Lower control limit for subgroup mean
`_LIMITN_`	Nominal sample size associated with the control limits
`_LSL_`	Lower specification limit
`_MEAN_`	Process mean ( $\overline{\overline{X}}$ or $\mu _{0}$ )
`_S_`	Value of central line on s chart
`_SIGMAS_`	Multiple (k) of standard error of $\bar{X}_{i}$ or $s_{i}$
`_STDDEV_`	Process standard deviation ( $\hat{\sigma }$ or $\sigma _{0}$ )
`_SUBGRP_`	Subgroup-variable specified in the XSCHART statement
`_TARGET_`	Target value
`_TYPE_`	Type (estimate or standard value) of `_MEAN_` and `_STDDEV_`
`_UCLS_`	Upper control limit for subgroup standard deviation
`_UCLX_`	Upper control limit for subgroup mean
`_USL_`	Upper specification limit
`_VAR_`	Process specified in the XSCHART statement

Notes:

If the control limits vary with subgroup sample size, the special missing value 'V' is assigned to the variables _LIMITN_, _LCLX_, _UCLX_, _LCLS_, _S_, and _UCLS_.
If the limits are defined in terms of a multiple k of the standard errors of $\bar{X_{i}}$ and $s_{i}$ , the value of _ALPHA_ is computed as $\alpha =2(1-\Phi (k))$ , where $\Phi (\cdot )$ is the standard normal distribution function.
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.
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.
Optional BY variables are saved in the OUTLIMITS= data set.

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

OUTHISTORY= Data Set

The OUTHISTORY= data set saves subgroup summary statistics. The following variables are saved:

the subgroup-variable
a subgroup mean variable named by process suffixed with X
a subgroup standard deviation variable named by process suffixed with S
a subgroup sample size variable named by process suffixed with N

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.

Subgroup summary variables are created for each process specified in the XSCHART statement. For example, consider the following statements:

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

The data set Summary contains variables named Lot, WidthX, WidthS, WidthN, DiameterX, DiameterS, and DiameterN. 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 Summary Statistics.

OUTTABLE= Data Set

The OUTTABLE= data set saves subgroup summary statistics, control limits, and related information. Table 18.67 lists the variables that are saved.

Table 18.67: OUTTABLE= Data Set Variables

Variable	Description
`_ALPHA_`	Probability ( $\alpha$ ) of exceeding control limits
`_EXLIM_`	Control limit exceeded on $\bar{X}$ chart
`_EXLIMS_`	Control limit exceeded on s chart
`_LCLS_`	Lower control limit for standard deviation
`_LCLX_`	Lower control limit for mean
`_LIMITN_`	Nominal sample size associated with the control limits
`_MEAN_`	Process mean
`_S_`	Average standard deviation
`_SIGMAS_`	Multiple (k) of the standard error associated with control limits
Subgroup	Values of the subgroup variable
`_SUBN_`	Subgroup sample size
`_SUBS_`	Subgroup standard deviation
`_SUBX_`	Subgroup mean
`_TESTS_`	Tests for special causes signaled on $\bar{X}$ chart
`_TESTS2_`	Tests for special causes signaled on s chart
`_UCLS_`	Upper control limit for standard deviation
`_UCLX_`	Upper control limit for mean
`_VAR_`	Process specified in the XSCHART 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:

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).
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.
The variable _TESTS2_ is saved if you specify the TESTS2= option.
The variables _EXLIM_, _EXLIMS_, _TESTS_, and _TESTS2_ 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.