Graphical Enhancements

Displaying Multiple Sets of Control Limits

See SHWCLMS in the SAS/QC Sample Library

This section describes the use of the READPHASES= and READINDEXES= options for creating Shewhart charts that display distinct sets of control limits for multiple phases of observations. The term phase refers to a group of consecutive observations in the input data set. For example, the phases might correspond to the time periods during which a new process was brought into production and then put through successive changes.

To display phases, your input data must include a character variable named _PHASE_, whose length cannot exceed 16. (If your data set does not include a variable named _PHASE_, you can temporarily rename another character variable to _PHASE_, as illustrated in the statements in Displaying Stratification in Phases. Each phase consists of a group of consecutive observations with the same value of _PHASE_.

To display distinct sets of predetermined control limits for the phases, you must provide the limits in a LIMITS= data set. This data set must include a character variable named _INDEX_, whose length cannot exceed 16. This variable identifies the sets of control limits (observations) in the LIMITS= data set that are to be associatied with the phases. This data set must also include a number of other variables with reserved names that begin and end with an underscore. The particular structure of a LIMITS= data set depends on the chart statement that you are using; for details, see the sections titled "LIMITS= Data Set" in the chapters for the various chart statements. In addition to specifying a LIMITS= data set, you must also specify the READINDEXES= and READPHASES= options in the chart statement.

Note: To display a single set of predetermined control limits with multiple phases, simply specify a LIMITS= data set in the procedure statement. If you are using Release 6.09 or an earlier release, you must also specify the READLIMITS option. The control limits are read from the first observation in the LIMITS= data for which the variable _VAR_ is equal to the name of the process and the variable _SUBGRP_ is equal to the name of the subgroup-variable. For an example, see "Reading Preestablished Control Limits" .

This section describes the combinations of the READINDEXES= and READPHASES= options that you can specify. The examples that follow use the HISTORY= data set FLANGE listed in Figure 47.7 and the LIMITS= data set FLANGLIM listed in Figure 47.8. The data in FLANGE consist of means and ranges of flange width measurements for subgroups of size five. The observations are grouped into three phases determined by the _PHASE_ values Production, Change 1, and Change 2. Three sets of control limits are provided in FLANGLIM, corresponding to the _INDEX_ values Start, Production, and Change 1.

Obs	_phase_	day	sample	flwidthx	flwidthr	flwidthn
1	Production	08FEB90	6	0.97360	0.06247	5
2	Production	09FEB90	7	1.00486	0.11478	5
3	Production	10FEB90	8	1.00251	0.13537	5
4	Production	11FEB90	9	0.95509	0.08378	5
5	Production	12FEB90	10	1.00348	0.09993	5
6	Production	15FEB90	11	1.02566	0.06766	5
7	Production	16FEB90	12	0.97053	0.07608	5
8	Production	17FEB90	13	0.94713	0.10170	5
9	Production	18FEB90	14	1.00377	0.04875	5
10	Production	19FEB90	15	0.99604	0.08242	5
11	Change 1	22FEB90	16	0.99218	0.09787	5
12	Change 1	23FEB90	17	0.99526	0.02017	5
13	Change 1	24FEB90	18	1.02235	0.10541	5
14	Change 1	25FEB90	19	0.99950	0.11476	5
15	Change 1	26FEB90	20	0.99271	0.05395	5
16	Change 1	01MAR90	21	0.98695	0.03833	5
17	Change 1	02MAR90	22	1.00969	0.06183	5
18	Change 1	03MAR90	23	0.98791	0.05836	5
19	Change 1	04MAR90	24	1.00170	0.05243	5
20	Change 1	05MAR90	25	1.00412	0.04815	5
21	Change 2	08MAR90	26	1.00261	0.05604	5
22	Change 2	09MAR90	27	0.99553	0.02818	5
23	Change 2	10MAR90	28	1.01463	0.05558	5
24	Change 2	11MAR90	29	0.99812	0.03648	5
25	Change 2	12MAR90	30	1.00047	0.04309	5
26	Change 2	15MAR90	31	0.99714	0.03689	5
27	Change 2	16MAR90	32	0.98642	0.04809	5
28	Change 2	17MAR90	33	0.98891	0.07777	5
29	Change 2	18MAR90	34	1.00087	0.06409	5
30	Change 2	19MAR90	35	1.00863	0.02649	5

Figure 47.7: Listing of the HISTORY= Data Set FLANGE

Obs	_index_	_var_	_subgrp_	_type_	_limitn_	_alpha_	_sigmas_	_lclx_	_mean_	_uclx_	_lclr_	_r_	_uclr_	_stddev_
1	Change 1	FLWIDTH	SAMPLE	ESTIMATE	5	.0026998	3	0.96167	0.99924	1.03680	0	0.06513	0.13771	0.028000
2	Production	FLWIDTH	SAMPLE	ESTIMATE	5	.0026998	3	0.93792	0.98827	1.03862	0	0.08729	0.18458	0.037530
3	Start	FLWIDTH	SAMPLE	ESTIMATE	5	.0026998	3	0.87088	0.96803	1.06517	0	0.16842	0.35612	0.072409

Figure 47.8: Listing of the LIMITS= Data Set FLANLIM

For each of the READINDEXES= and READPHASES= options, you can specify a single value, a list of values, or the keyword ALL. You can also leave these options unspecified. Thus, there are 16 possible combinations of specifications for the two options, as explained by the following table and notes. The two most commonly encountered combinations are

reading a single set of limits for one or more phases (see Note 1)
reading a set of limits matched with a set of phases (see Note 4)

Table 47.1: Combinations of READPHASES= and READINDEXES= Specifications

	READPHASES=
READINDEXES=	Single Value	Multiple Values	Keyword ALL	Not Specified
Single Value	See Note 1	See Note 1	See Note 2	See Note 3
Multiple Values	See Note 9	See Note 4	See Note 2	See Note 2
Keyword ALL	See Note 5	See Note 5	See Note 6	See Note 6
Not Specified	See Note 7	See Note 7	See Note 8	See Note 8

READPHASES=value|value-list and READINDEXES=value
The only phases (groups of observations) read are those for which _PHASE_ equals one of the values specified with the READPHASES= option. The chart displays a single set of control limits given by the first observation in the LIMITS= data set for which _INDEX_ is equal to the READINDEXES= value.
For example, the following statements create a chart for the phases Change 1 and Change 2, with control limits read from the second observation in FLANGLIM. The chart is displayed in Figure 47.9.
```
   proc shewhart history=flange limits=flanlim;
      xchart flwidth*sample /
         readphase = ('Change 1' 'Change 2')
         readindex = ('Production')
         phaseref
         phaselegend
         cinfill   = ywh
         cframe    = vibg
         cconnect  = salmon;
   run;
```
Figure 47.9: A Single Set of Control Limits for Multiple Phases
READPHASES=ALL and READINDEXES=value|value-list or READPHASES= is omitted and READINDEXES=value-list
The only phases read are those for which _PHASE_ equals one of the values specified with the READINDEXES= option. The chart displays a different set of control limits for each phase, read from the first observation in the LIMITS= data set for which _INDEX_ is equal to the corresponding value.
For example, the following statements create a chart for the phases Production and Change 1 with control limits read from the second and first observations in FLANGLIM, respectively. The chart is displayed in Figure 47.10.
```
   proc shewhart history=flange limits=flanlim;
      xchart flwidth*sample /
         readphase = all
         readindex = ('Production' 'Change 1')
         phaseref
         phaselegend
         cframe    = vibg
         cinfill   = ywh
         cconnect  = salmon;
   run;
```
If you wish to specify a single set of control limits to use with all the phases, use the READINDEXES= option without the READPHASES= option (see Note 3).

Figure 47.10: READPHASES=ALL with a List of Values for READINDEXES=
READPHASES= is omitted and READINDEXES=value
All observations are read from the input data set. The chart displays a single set of control limits read from the first observation in the LIMITS= data for which _INDEX_ equals the value.
READPHASES=value-list and READINDEXES=value-list
The only phases read are those for which _PHASE_ equals one of the values specified with the READPHASES= option. The chart displays a different set of control limits for each phase, given by the first observation in the LIMITS= data set for which _INDEX_ equals the READINDEXES=value. Control limits are matched with phases in the order listed.
For example, the following statements create a chart for the phases Production and Change 1 with control limits read from the first and second observations in FLANGLIM, respectively. The chart produced by these statements is identical to the chart in Figure 47.10.
```
   proc shewhart history=flange limits=flanlim;
     xchart flwidth*sample /
         readphases  = ('Production' 'Change 1')
         readindexes = ('Production' 'Change 1')
         phaseref
         phaselegend
         cframe      = vibg
         cinfill     = ywh
         cconnect    = salmon;
   run;
```
The order of the READINDEX=value-list is critical. For instance, the previous statements with READINDEXES=('Change 1' 'Production') create the chart in Figure 47.11, in which the control limits are mismatched with the phases.

Figure 47.11: Multiple Phases with Mismatched Control Limits
READPHASES=value|value-list and READINDEXES=ALL
The only phases read are those for which _PHASE_ equals one of the values specified with the READPHASES= option. The chart displays a different set of control limits for each phase, read from the first observation in the LIMITS= data set for which _INDEX_ equals the value corresponding to the phase.
For example, the following statements create a chart for the phases Production and Change 1 with the control limits read from the second and first observations in FLANGLIM, respectively:
```
   proc shewhart history=flange limits=flanlim;
      xchart flwidth*sample /
         readphases  = ('Production' 'Change 1')
         readindexes = all
         phaseref
         phaselegend ;
   run;
```
The chart is identical to the chart in Figure 47.10. In general, to read a set of phases with identically labeled control limits, you can specify the phases with either the READPHASES= or READINDEXES= option, and you can specify the keyword ALL with the other option.
READPHASES=ALL and READINDEXES=ALL or READPHASES= is omitted and READINDEXES=ALL
All phases are read for which _PHASE_ is a value of _INDEX_ in the LIMITS= data set. The chart displays a different set of control limits for each phase, read from the first observation in the LIMITS= data set for which _INDEX_ equals the value of _PHASE_.
For example, the following statements create a chart for the phases Production and Change 1 with control limits read from the second and first observations in FLANGLIM, respectively. These two phases are read because they are the only phases in FLANGE with matching _INDEX_ values in FLANLIM. The chart is identical to that in Figure 47.10.
```
   proc shewhart history=flange limits=flanlim;
      xchart flwidth*sample /
         readphase = all
         readindex = all
         phaseref
         phaselegend ;
   run;
```
Note that an identical chart would be produced if you were to omit the READPHASES= option.
READPHASES=value|value-list and READINDEXES= is omitted
The only phases read are those for which _PHASE_ equals one of the values specified with the READPHASES= option. The chart displays a single set of control limits read from the first observation in the LIMITS= data set for which _VAR_ equals the process and _SUBGRP_ equals the name of the subgroup-variable specified in the chart statement.
For example, the following statements create a chart for the phases Production and Change 1 with control limits read from the first observation in FLANGLIM, because this is the first observation for which _VAR_ equals FLWIDTH and _SUBGRP_ equals SAMPLE.
```
   proc shewhart history=flange limits=flanlim;
      xchart flwidth*sample /
         readphases  = ('Production' 'Change 1')
         phaseref
         phaselegend
         cframe      = vibg
         cinfill     = ywh
         cconnect    = salmon;
   run;
```
The chart is displayed in Figure 47.12.

Figure 47.12: Value-list for READPHASES= with READINDEXES= Omitted
READPHASES=ALL and READINDEXES= is omitted or READPHASES= is omitted and READINDEXES= is omitted
All observations are read from the input data set. The chart displays a single set of control limits read from the first observation in the LIMITS= data set for which _VAR_ equals the process and _SUBGRP_ equals the name of the subgroup-variable specified in the chart statement.
For example, the following statements create a chart for all the phases in FLANGE with control limits read from the first observation in FLANGLIM, because this is the first observation for which _VAR_ equals FLWIDTH and _SUBGRP_ equals SAMPLE:
```
   proc shewhart history=flange limits=flanlim;
      xchart flwidth*sample /
         readphase = all
         phaseref
         phaselegend
         cframe    = vibg
         cinfill   = ywh
         cconnect  = salmon;
   run;
```
The chart is shown in Figure 47.13. Note that an identical chart would be produced if you were to omit the READPHASES= option (except that the phase reference lines and phase legends would be omitted).

Figure 47.13: READPHASES=ALL with READINDEXES= Omitted
READPHASES=value and READINDEXES=value-list
The procedure generates an error message.

The following tables summarize the various combinations of the READPHASES= and READINDEXES= options that you can specify.

Table 47.2: READINDEXES=index-value

READPHASES=	Phases Displayed	Control Limits Displayed
phase-value	_PHASE_ = phase-value	_INDEX_ = index-value
phase-value list	_PHASE_ = phase-value list	_INDEX_ = index-value
Keyword ALL	_PHASE_ = index-value	_INDEX_ = index-value
Not Specified	All phases	_INDEX_ = index-value

Table 47.3: READINDEXES=index-value list

READPHASES=	Phases Displayed	Control Limits Displayed
phase-value	No chart displayed	No chart displayed
phase-value list	_PHASE_ = phase-value list	_INDEX_ = index-value list with control limits matched to phases in the order listed
Keyword ALL	_PHASE_ = index-value list	_INDEX_ = index-value list
Not Specified	_PHASE_ = index-value list	_INDEX_ = index-value list

Table 47.4: READINDEXES=ALL

READPHASES=	Phases Displayed	Control Limits Displayed
phase-value	_PHASE_ = phase-value	_INDEX_ = phase-value
phase-value list	_PHASE_ = phase-value list	_INDEX_ = phase-value list
Keyword ALL	_PHASE_ = _INDEX_	_INDEX_ = _PHASE_
Not Specified	_PHASE_ = _INDEX_	_INDEX_ = _PHASE_

Table 47.5: READINDEXES= Not Specified

READPHASES=	Phases Displayed	Control Limits Displayed
phase-value	_PHASE_ = phase-value	First LIMITS= observation for which _VAR_=process name and _SUBGRP_=subgroup-variable name
phase-value list	_PHASE_ = phase-value list	same as previous entry
Keyword ALL	All phases	same as previous entry
Not Specified	All phases	same as previous entry

Chapter Contents
Previous
Next
Top