P control chart, answers to on your own exercises

Steve Simon

2007-03-07

On the web page

you were asked to calculate P charts for the following two data sets:

Worker #2: 60 56 78 60 74 70 70 68 82 76 72 76 68 78 76 68 74 56 74 62 60 70 60 84 Worker #3: 24 32 20 14 32 22 20 26 26 38 30 16 36 32 38 14 26 14 38 20 14 18 22 34

For worker #2, the average of all 24 proportions is 0.6967. The control limits are calculated as

and the control chart is

Notice that the computer rounds these numbers slightly differently, reporting the lower control limit as 0.50 rather than 0.51. Do not worry about these small differences, but the computer’s value is slightly more accurate.

There are no points out of control on this chart.

For worker #3, the average proportion is 0.2525. The control limits are calculated as

and the control chart is

Again, the answers diverge slightly because of rounding. There are no points out of control on this chart.