Control Charts for Attributes. p chart (fraction of nonconforming items) np chart ( number of nonconforming items) c chart (number of nonconformities in some unit) u chart (number of nonconformities per unit). The p Chart Procedure.
Step1. Gather the data. Select the size, frequency and number of samples.
Step2. Calculate p for each sample.
Step 3. Set the scale for the control chart.
Step 4. Plot the p values and connect them.
Step 5. Calculate p-bar, and the control limits after about 20 points have been charted.
Step 6. Interpret the chart
UCL = np0 +3np0 (1-p0)
LCL = np0 - 3np0 (1-p0)
UCL = x + 3 x (1- x/n)
LCL = x - 3 x (1- x/n)
UCL = c0 + 3 c0
LCL = c0 - 3 c0
UCL = c+ 3c
LCL = c- 3c
u = c/k k is the number of inspection
units in each sample
UCL = u0 + 3 u0/k
LCL = u0 - 3 u0/k