Input Data Sets

EWMACHART Statement

Input Data Sets

DATA= Data Set

You can read raw data (process measurements) from a DATA= data set specified in the PROC MACONTROL statement. Each process specified in the EWMACHART 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 EWMACHART 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 n_i items, there should be n_i 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 MACONTROL procedure reads all the observations in a DATA= data set. However, if the data set includes the variable _PHASE_, you can read selected groups of observations (referred to as phases) with the READPHASES= option (for an example, see "Displaying Stratification in Phases" ).

For an example of a DATA= data set, see "Creating EWMA Charts from Raw Data" .

LIMITS= Data Set

You can read preestablished control limit parameters from a LIMITS= data set specified in the PROC MACONTROL statement. The LIMITS= data set used by the MACONTROL procedure does not contain the actual control limits, but rather it contains the parameters required to compute the limits. For example, the following statements read parameters from the data set PARMS:^*

   proc macontrol data=parts limits=parms;
      ewmachart gap*day;
   run;

The LIMITS= data set can be an OUTLIMITS= data set that was created in a previous run of the MACONTROL procedure. Such data sets always contain the variables required for a LIMITS= data set; see "OUTLIMITS= 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 the variable _WEIGHT_, which specifies the weight parameter used to compute the EWMAs. 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, STDMEAN, and STDSIGMA.
BY variables are required if specified with a BY statement.

Some advantages of working with a LIMITS= data set are that

it facilitates reusing a permanently saved set of parameters
a distinct set of parameters can be read for each process specified in the EWMACHART statement
it facilitates keeping track of multiple sets of parameters that accumulate for the same process as the process evolves over time

For an example, see "Reading Preestablished Control Limit Parameters" .

HISTORY= Data Set

You can read subgroup summary statistics from a HISTORY= data set specified in the PROC MACONTROL statement. This allows you to reuse OUTHISTORY= data sets that have been created in previous runs of the MACONTROL, SHEWHART, or CUSUM procedures or to read output data sets created with SAS summarization procedures such as PROC MEANS.

A HISTORY= data set used with the EWMACHART statement must contain the following:

the subgroup-variable
a subgroup mean variable for each process
a subgroup sample size variable for each process
a subgroup standard deviation 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 suffix characters X, S, and N, respectively.

For example, consider the following statements:

   proc macontrol history=cliphist;
      ewmachart (gap diameter)*day  / weight=0.2;
   run;

The data set CLIPHIST must include the variables DAY, GAPX, GAPS, GAPN, DIAMTERX, DIAMTERS, and DIAMTERN.

Although a subgroup EWMA variable (named by the process name suffixed with E) is saved in an OUTHISTORY= data set, it is not required in a HISTORY= data set, because the subgroup mean variable is sufficient to compute the EWMAs.

Note that, if you specify a process name that contains eight characters, the names of the 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 MACONTROL procedure reads all the observations in a HISTORY= data set. However, if the HISTORY= 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 EWMA Charts from Subgroup Summary Data" .

TABLE= Data Set

You can read summary statistics and control limits from a TABLE= data set specified in the PROC MACONTROL statement. This enables you to reuse an OUTTABLE= data set created in a previous run of the MACONTROL procedure.

The following table lists the variables required in a TABLE= data set used with the EWMACHART statement:

Variable Description

_EWMA_ exponentially weighted moving average

_LCLE_ lower control limit for EWMA

_LIMITN_ nominal sample size associated with the control limits

_MEAN_ process mean

subgroup-variable values of the subgroup-variable

_SUBN_ subgroup sample size

_SUBS_ subgroup standard deviation

_SUBX_ subgroup mean

_UCLE_ upper control limit for EWMA

_WEIGHT_ weight (r) assigned to most recent subgroup mean in computation of EWMA

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.
_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 Limit Parameters" .

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Variable	Description
_EWMA_	exponentially weighted moving average
_LCLE_	lower control limit for EWMA
_LIMITN_	nominal sample size associated with the control limits
_MEAN_	process mean
subgroup-variable	values of the subgroup-variable
_SUBN_	subgroup sample size
_SUBS_	subgroup standard deviation
_SUBX_	subgroup mean
_UCLE_	upper control limit for EWMA
_WEIGHT_	weight (r) assigned to most recent subgroup mean in computation of EWMA