1. P Chart Control Chart The P chart is a specialized chart
Think “P” for Proportion or Percentage
But, it must be based upon individual go / no-go trials (like flipping a coin)
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2. Excel Spreadsheet Data
Open file P_chart.xls
This is the percent of safety inspections identifying Personal Protective Equipment issues
It is real data.
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3. The data
Note that we track the individual numbers of inspections with PPE issues, and also the total number of inspections.
Don’t just record the percentages, record both numbers.
Note that one safety inspection either had a PPE issue or did not (heads / tails).
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4. Calculate the Baseline Average
Calculate the baseline averages by summing up the numerators, and summing up the denominators, and then divide.
Apply this as a horizontal line on the chart.
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5. Calculate the Standard Deviation
For this chart, we use the binomial standard deviation
That is equal to the square root of the average times one minus the average, divided by the number of trials
= sqrt ( pbar * ( 1 – pbar ) / n )
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6. The UCL and LCL and vary! Note that the UCL and LCL lines “sawtooth”.
The more trials, the closer to the average, the less, the farther away.
3 heads in 10 coin flips is “okay”, but 30 in 100 is not. Note both are 30% heads.
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7. Warning
Just because the data are percentages does not imply it is a p chart.
There must be individual trials, with only one of two results possible
Inappropriate for p charts:
Percent of budget spent
Percent of weight of material defective
Percent change from last month to this month
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