1. Savannah River Nuclear Solutions, LLC
P Chart Control Charts
Steven S Prevette
Senior Statistician
Savannah River Nuclear Solutions, LLC
SPC Trending Primer/ Two Day Training

2. P Charts 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)

3. Exercise Description Open file P_chart.xls
This is the percent of safety inspections identifying Personal Protective Equipment issues
It is real data.

4. The Data
Note that the individual numbers of inspections with PPE issues were tracked, and also the total number of inspections.
Don’t just record the percentages, record both numbers.
Note that each individual safety inspection either had a PPE issue or did not (heads / tails).

5. 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.

6. 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 (pbar), divided by the number of trials (n)
= sqrt ( pbar * ( 1 – pbar ) / n )

7. The UCL and LCL Vary! Note that the UCL and LCL lines “sawtooth”.
The more trials, the closer to the average, the less, the farther away.
Physical Example:
3 heads in 10 coin flips is “okay”
30 in 100 is not
Note both are 30% heads, but differing number of coin flips (n)

8. 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