NPCHART Statement: SHEWHART Procedure

Output Data Sets

OUTLIMITS= Data Set

The OUTLIMITS= data set saves control limits and control limit parameters. Table 18.33 lists the variables that can be saved.

Table 18.33: OUTLIMITS= Data Set

Variable

Description

_ALPHA_

Probability ($\alpha $) of exceeding limits

_INDEX_

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

_LCLNP_

Lower control limit for number of nonconforming items

_LIMITN_

Sample size associated with the control limits

_NP_

Average number of nonconforming items ($n_{i}\bar{p}$ or $n_{i}p_{0}$)

_P_

Average proportion of nonconforming items ($\bar{p}$ or $p_{0}$)

_SIGMAS_

Multiple (k) of standard error of $X_ i$

_SUBGRP_

Subgroup-variable specified in the NPCHART statement

_TYPE_

Type (standard or estimate) of _NP_

_UCLNP_

Upper control limit for number of nonconforming items

_VAR_

Process specified in the NPCHART statement


Notes:

  1. If the control limits vary with subgroup sample size, the special missing value V is assigned to the variables _LIMITN_, _LCLNP_, _UCLNP_, _NP_, and _SIGMAS_.

  2. If the limits are defined in terms of a multiple k of the standard error of $X_{i}$, the value of _ALPHA_ is computed as $\alpha = P\{ X_ i < \mr{\_ LCLNP\_ }\} + P\{ X_ i > \mr{\_ UCLNP\_ }\} $, using the incomplete beta function.

  3. If the limits are probability limits, the value of _SIGMAS_ is computed as $k=(\mr{\_ UCLNP\_ } - \mr{\_ NP\_ })/ \sqrt {\mr{\_ NP\_ }(1-\mr{\_ NP\_ }) /\mr{\_ LIMITN\_ }}$. If _LIMITN_ has the special missing value V, this value is assigned to _SIGMAS_.

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

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

  • the subgroup proportion of nonconforming items variable named by the process suffixed with P

  • a subgroup sample size variable named by the 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 NPCHART statement. For example, consider the following statements: 

proc shewhart data=Input;
   npchart (Rework Rejected)*Batch / outhistory=Summary
                                     subgroupn =30;
run;

The data set Summary contains variables named Batch, ReworkP, ReworkN, RejectedP, and RejectedN.

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 Proportions of Nonconforming Items.

OUTTABLE= Data Set

The OUTTABLE= data set saves subgroup summary statistics, control limits, and related information. Table 18.34 lists the variables that are saved.

Table 18.34: OUTTABLE= Data Set Variables

Variable

Description

_ALPHA_

Probability ($\alpha $) of exceeding control limits

_EXLIM_

Control limit exceeded on $np$ chart

_LCLNP_

Lower control limit for number of nonconforming items

_LIMITN_

Nominal sample size associated with the control limits

_NP_

Average number of nonconforming items

_SIGMAS_

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

Subgroup

Values of the subgroup variable

_SUBNP_

Subgroup number of nonconforming items

_SUBN_

Subgroup sample size

_TESTS_

Tests for special causes signaled on $np$ chart

_UCLNP_

Upper control limit for number of nonconforming items

_VAR_

Process specified in the NPCHART 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 the first four tests (the tests appropriate for $np$ charts) and Tests 2 and 4 are positive for a given subgroup, the value of _TESTS_ has a 2 for the second character, a 4 for the fourth character, and blanks for the other six characters.

  3. The variables _EXLIM_ 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.