SCHART Statement: SHEWHART Procedure

OUTLIMITS= Data Set

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

Notes:

1. The variables _LCLX_, _MEAN_, and _UCLX_ are saved to enable the OUTLIMITS= data set to be used as a LIMITS= data set with the BOXCHART, XCHART, and XSCHART statements.

2. 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_.

3. If the limits are defined in terms of a multiple of the standard error of , the value of _ALPHA_ is computed as

   where is the cumulative distribution function of the standard deviation of a sample of observations from a normal population with unit standard deviation, and is the value of _LIMITN_. If _LIMITN_ has the special missing value V, this value is assigned to _ALPHA_.

4. If the limits are probability limits, the value of _SIGMAS_ is computed as , where is the standard error of the standard deviation of observations from a normal population with unit standard deviation. If _LIMITN_ has the special missing value V, this value is assigned to _SIGMAS_.

5. 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.

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

The OUTLIMITS= data set contains one observation for each process specified in the SCHART 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

The subgroup mean variable is included so that the data set can be reused as a HISTORY= data set with the BOXCHART, XCHART, and XSCHART statements, as well as the SCHART statement.

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 SCHART statement. For example, consider the following statements:

proc shewhart data=Steel;
   schart (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. The following variables are saved:

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= option, respectively, or with the corresponding variables in a LIMITS= data set).

2. The variable _TESTS2_ is saved if you specify the TESTS2= option.

3. The variables _EXLIM_ 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.