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NPCHART Statement |
See SHWNP1 in the SAS/QC Sample Library |
The previous example illustrates how you can create np charts using raw data (counts of nonconforming items). However, in many applications, the data are provided in summarized form as proportions or percentages of nonconforming items. This example illustrates how you can use the NPCHART statement with data of this type.
The following data set provides the data from the preceding
example in summarized form:
data cirprop; input batch pfailed @@; sampsize=500; datalines; 1 0.010 2 0.012 3 0.022 4 0.012 5 0.008 6 0.018 7 0.034 8 0.020 9 0.024 10 0.018 11 0.016 12 0.014 13 0.014 14 0.030 15 0.016 16 0.036 17 0.024 18 0.032 19 0.008 20 0.014 21 0.034 22 0.024 23 0.016 24 0.014 25 0.030 26 0.012 27 0.016 28 0.024 29 0.014 30 0.018 ;
A listing of CIRPROP is shown in Figure 37.3.
The subgroups are still indexed by BATCH. The variable
PFAILED contains the proportions of nonconforming items, and
the variable SAMPSIZE contains the subgroup sample sizes.
The following statements create an np chart
identical to the one in Figure 37.2:
title 'np Chart for the Number of Failing Circuits'; symbol v=dot; proc shewhart data=cirprop; npchart pfailed*batch / subgroupn=sampsize dataunit =proportion; label pfailed = 'Number of FAIL'; run;
The DATAUNIT= option specifies that the values of the
process (PFAILED) are proportions of nonconforming
items. By default, the values of the process are
assumed to be counts of nonconforming items (see
the previous example).
Alternatively, you can read the data set CIRPROP
by specifying it
as a HISTORY= data set in the PROC SHEWHART statement.
A HISTORY= data set used with the NPCHART
statement must contain the following variables:
Furthermore, the names of the subgroup proportion and sample size variables must begin with the process name specified in the NPCHART statement and end with the special suffix characters P and N, respectively.
To specify CIRPROP
as a HISTORY= data set and FAIL as the process,
you must rename the variables PFAILED and SAMPSIZE
to FAILP and FAILN, respectively.
The following statements
temporarily rename PFAILED and SAMPSIZE for the duration
of the procedure step:
title 'np Chart for the Number of Failing Circuits'; proc shewhart history=cirprop lineprinter (rename=(pfailed =failp sampsize=failn )); npchart fail*batch='*'; run;
The resulting np chart is shown in Figure 37.4.
Since the LINEPRINTER option is specified in the PROC
SHEWHART statement, line printer output is produced.
The asterisk specified in single quotes after the
subgroup-variable indicates the character used to plot
points. This character must follow an equal sign.
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In this example, it is more convenient to use CIRPROP as a DATA= data set than as a HISTORY= data set. As illustrated in the next example, it is generally more convenient to use the HISTORY= option for input data sets that have been created previously by the SHEWHART procedure as OUTHISTORY= data sets.
For more information, see "HISTORY= Data Set" .
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