Input Data Sets
You can read raw data (process measurements) from a DATA= data
set specified in the PROC SHEWHART statement.
Each process specified in the XSCHART
statement must be a SAS variable in the DATA= data set.
This variable provides measurements that must be grouped
into subgroup samples indexed by the subgroup-variable.
The subgroup-variable, which is specified in the
XSCHART statement,
must also be a SAS variable in the DATA= data set.
Each observation in a DATA= data set
must contain a value for each process
and a value for the subgroup-variable.
If the
i th subgroup contains ni items,
there should be ni
consecutive observations for which the value of the subgroup variable
is the index of the i th subgroup.
For example, if each subgroup
contains five items and there are 30 subgroup samples, the
DATA= data set should contain 150 observations.
Other variables that can be read from a DATA= data set include
- _PHASE_ (if the READPHASES= option is specified)
- block-variables
- symbol-variable
- BY variables
- ID variables
By default, the SHEWHART procedure reads all the observations in a DATA=
data set. However, if the DATA= data set includes the variable
_PHASE_, you can read selected groups of observations (referred to
as phases) by specifying the READPHASES= option
(for an example, see "Displaying Stratification in Phases"
).
For an example of a DATA= data set, see
"Creating Charts for Means and Standard Deviations from Raw Data" .
You can read preestablished control limits (or parameters from
which the control limits can be calculated) from a LIMITS=
data set specified in the PROC SHEWHART statement.
For example, the following
statements read control limit information from the data set
CONLIMS:*
proc shewhart data=info limits=conlims;
xschart weight*batch;
run;
The LIMITS= data set can be an OUTLIMITS= data set that was created
in a previous run of the SHEWHART procedure. Such data sets always
contain the variables required for a LIMITS= data set. The LIMITS=
data set can also be created directly using a DATA step.
When you create a LIMITS= data set, you must provide
one of the following:
- the variables _LCLX_, _MEAN_, _UCLX_, _LCLS_, _S_, and
_UCLS_, which specify the control limits directly
- the variables _MEAN_ and _STDDEV_, which are used
to calculate the control limits according to the
equations in Table 44.22
In addition, note the following:
- The variables _VAR_ and _SUBGRP_ are required.
These must be character variables of length 8.
- The variable _INDEX_ is required if you specify the
READINDEX= option; this must be a character variable
of length 16.
- The variables _LIMITN_, _SIGMAS_ (or _ALPHA_), and
_TYPE_ are optional, but they are recommended to maintain
a complete set of control limit information.
The variable _TYPE_ must be a character variable of length
8; valid values are ESTIMATE, STANDARD,
STDMU, and STDSIGMA.
- BY variables are required if specified with a BY statement.
For an example, see "Reading Preestablished Control Limits" .
You can read subgroup summary statistics from a HISTORY=
data set specified in the PROC SHEWHART statement.
This allows you to reuse OUTHISTORY= data sets that have been
created in previous runs of the SHEWHART, CUSUM, or MACONTROL
procedures or to read output data sets created with
SAS summarization procedures, such as PROC MEANS.
A HISTORY= data set used with the XSCHART statement
must contain the following variables:
- the subgroup-variable
- a subgroup mean variable for each process
- a subgroup standard deviation variable for each process
- a subgroup sample size variable for each process
The names of the subgroup mean, subgroup standard deviation, and subgroup
sample size variables must be the process name concatenated with
the special suffix characters X, S, and N, respectively.
For example, consider the following statements:
proc shewhart history=summary;
xschart (weight yldstren)*batch;
run;
The data set SUMMARY must include the variables BATCH, WEIGHTX,
WEIGHTS, WEIGHTN, YLDSRENX, YLDSRENS, and YLDSRENN.
Note that if you specify a process name that contains eight
characters, the names of
summary variables must
be formed from the first four
characters and the last three characters of the process
name, suffixed with the appropriate character.
Other variables that can be read from a HISTORY= data set include
- _PHASE_ (if the READPHASES= option is specified)
- block-variables
- symbol-variable
- BY variables
- ID variables
By default, the SHEWHART procedure reads all of the observations in a HISTORY=
data set. However, if the data set includes the
variable _PHASE_, you can read selected groups of observations
(referred to as phases)
by specifying the READPHASES= option
(see "Displaying Stratification in Phases"
for an example).
For an example of a HISTORY= data set,
see "Creating Charts for Means and Standard Deviations from Summary Data" .
You can read summary statistics and control limits from a
TABLE= data set specified in the PROC SHEWHART statement.
This enables you to reuse an OUTTABLE= data set created
in a previous run of the SHEWHART procedure.
Because the SHEWHART procedure
simply displays the information read from a TABLE= data set, you can
use TABLE= data sets to create specialized control charts.
Examples are provided in Chapter 49, "Specialized Control Charts."
The following table lists the variables required in a TABLE= data set
used with the XSCHART statement:
Table 44.25: Variables Required in a TABLE= Data Set
|
Variable
|
Description
|
| _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 |
| subgroup-variable | values of the subgroup-variable |
| _SUBN_ | subgroup sample size |
| _SUBS_ | subgroup standard deviation |
| _SUBX_ | subgroup mean |
| _UCLS_ | upper control limit for standard deviation |
| _UCLX_ | upper control limit for mean |
Other variables that can be read from a TABLE= data set include
- block-variables
- symbol-variable
- BY variables
- ID variables
- _PHASE_ (if the READPHASES= option is specified).
This variable must be a character variable of length 16.
- _TESTS_ (if the TESTS= option is specified). This variable
is used to flag tests for special causes
for subgroup means and must be
a character variable of length 8.
- _TESTS2_ (if the TESTS2= option is specified). This variable
is used to flag tests for special causes
for subgroup standard deviations and must be
a character variable of length 8.
- _VAR_. This variable is
required if more than one process is specified
or if the data set contains information for more
than one process. This variable must be a character
variable of length 8.
For an example of a TABLE= data set, see "Saving Control Limits"
.
Copyright © 1999 by SAS Institute Inc., Cary, NC, USA. All rights reserved.
