The SPC Procedure
Input Data Tables
DATA= Data Table
The SPC procedure accepts a primary input data table that you specify in the DATA= option in the PROC SPC statement. This data table contains measurements of one or more process quality characteristics. Data to be analyzed by using the CCHART, IRCHART, NPCHART, PCHART, or UCHART statement should contain one observation per subgroup. Data to be analyzed by using other chart statements usually contain multiple observations per subgroup.
The DATA= data table must contain the following variables:
the process name variable, which contains the names of the process variables. You specify it in the PROCESSNAME= option in the PROC SPC statement.
the process value variable, which contains the process measurements. You specify it in the PROCESSVALUE= option in the PROC SPC statement.
the subgroup name variables, which contains the names of the subgroup variables. You specify it in the SUBGROUPNAME= option in the PROC SPC statement.
the subgroup value variable, which contains the subgroup values. You specify it in the SUBGROUPVALUE= option in the PROC SPC statement.
An input data table that contains attribute data (see the section Classification of Shewhart Charts) can also include a variable that contains the number of inspection units per subgroup. You specify this variable in the SUBGROUPN= option in the chart statement. It is required in the NPCHART, PCHART, and UCHART statements and optional in the CCHART statement.
The DATA= data table can also contain BY variables (see the section BY Statement).
LIMITS= Data Table
You can read preestablished control limits or parameters from which the control limits can be calculated from a LIMITS= data table that you specify by using the LIMITS= option in the PROC SPC statement. The LIMITS= data table can be an OUTLIMITS= data table that was created in a previous run of the SPC procedure. Such data tables always contain the variables that a LIMITS= data table requires. For more information about the variables that are included in an OUTLIMITS= data table, see the section OUTLIMITS= Data Table.
The LIMITS= data table must contain the following variables:
the process name variable, which contains the names of the process variables. You specify it in the PROCESSNAME= option in the PROC SPC statement.
the subgroup name variable, which contains the names of the subgroup variables. You specify it in the SUBGROUPNAME= option in the PROC SPC statement.
For each control chart type, Table 20.25 lists the variables that are required to specify parameters from which control limits are calculated and the variables that contain preestablished control limits.
Table 20.25: Variables in the LIMITS= Data Table
Chart Type | Parameters | Limits |
|---|---|---|
c | | |
Individual measurements | | |
Moving range | | |
Medians | | |
| | |
p | | |
R | | |
s | | |
u | | |
| |
Notes:
You can include the variable
_LIMITN_, with or without parameters and limits variables, to specify different nominal subgroup sizes for different processes.The special missing value V indicates variables that vary with subgroup sample size. Their values are calculated from the data. Table 20.27 identifies the variables that are assigned the missing value V in an OUTLIMITS= data table when subgroup sample sizes vary.
The variables
_SIGMAS_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'.If you specify a BY statement, the LIMITS= data table must also contain the specified BY variables.
SPECS= Data Table
You can read process specification limits from which capability indices can be calculated from a SPECS= data table that you specify by using the SPECS= option in a chart statement for variables.
The SPECS= data table must include the process name variable, which, as the name suggests, contains the names of the process variables. You specify it in the PROCESSNAME= option in the PROC SPC statement. Table 20.26 lists the specification limit variables that the data table can include.
Table 20.26: SPECS= Data Table Variables
Variable | Description |
|---|---|
| Lower specification limit |
| Target value |
| Upper specification limit |
PROC SPC computes capability indices for each process that is included in the DATA= data table and whose specification limits are provided in the SPECS= data table. For more information about how capability indices are computed, see the section Capability Indices.