P Chart Control Chart The P chart is

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) • http: //www. hanford. gov/safety/vpp/pchart. htm SS Prevette

Excel Spreadsheet Data • Open file P_chart. xls • This is the percent of safety inspections identifying Personal Protective Equipment issues • It is real data. SS Prevette

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). SS Prevette

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. SS Prevette

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 ) SS Prevette

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. SS Prevette

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 SS Prevette