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 QQPLOT Statement

## Creating a Normal Quantile-Quantile Plot

 See CAPQQ1 in the SAS/QC Sample Library

Measurements of the distance between two holes cut into 50 steel sheets are saved as values of the variable DISTANCE in the following data set:

```   data sheets;
input distance @@;
label distance='Hole Distance in cm';
datalines;
9.80 10.20 10.27  9.70  9.76
10.11 10.24 10.20 10.24  9.63
9.99  9.78 10.10 10.21 10.00
9.96  9.79 10.08  9.79 10.06
10.10  9.95  9.84 10.11  9.93
10.56 10.47  9.42 10.44 10.16
10.11 10.36  9.94  9.77  9.36
9.89  9.62 10.05  9.72  9.82
9.99 10.16 10.58 10.70  9.54
10.31 10.07 10.33  9.98 10.15
;
```

The cutting process is in control, and you decide to check whether the process distribution is normal. The following statements create a Q-Q plot for DISTANCE, shown in Figure 10.1, with lower and upper specification lines at 9.5 cm and 10.5 cm:*

```   title 'Normal Quantile-Quantile Plot for Hole Distance';
legend1 frame cframe=ligr cborder=black position=center;
proc capability data=sheets noprint;
spec lsl=9.5   clsl=red
usl=10.5  cusl=blue;
qqplot distance / cframe = ligr
legend = legend1;
run;
```

The plot compares the ordered values of DISTANCE with quantiles of the normal distribution. The linearity of the point pattern indicates that the measurements are normally distributed. Note that a normal Q-Q plot is created by default. If you specify the LINEPRINTER option in the PROC CAPABILITY statement, the plot is created using a line printer, as shown in Figure 10.2. The specification lines are requested with the LSL= and USL= options in the SPEC statement.

Figure 10.1: Normal Quantile-Quantile Plot Created with Graphics Device

 Normal Quantile-Quantile Plot for Hole Distance

 ``` ------------------------------------------------------------------- H 10.75 + | o | + | l | + + | e 10.50 +UUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUUU+UUUUUUUUUUUUUUUUU| | + | D | +++ | i 10.25 + +++ | s | +++ | t | +++++ | a 10.00 + +++ | n | ++ | c | ++ | e 9.75 + +++++ | | + + | i | + | n 9.50 +LLLLLLLLLLLLLLL+LLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLL| | + | c | + | m 9.25 + | +----------+----------+----------+----------+----------+----------+ -3 -2 -1 0 1 2 3 Normal Quantiles Specifications: LLL Lower = 9.5 UUU Upper = 10.5 Observations: + (13 Hidden) ```
Figure 10.2: Normal Quantile-Quantile Plot Created with Line Printer

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