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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.

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Control charts for attributes
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
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


Chart interpretation
Chart Interpretation

  • Shifts. A shift of seven or more points to a higher or lower level could mean that something has affected the proportion defective or that the definition of what is defective has somehow changed.

  • Trends. A run of seven or more points up or down indicates that something is causing the proportion that is defective to change in a gradual manner, poorly trained operators, different inspection criteria, or poorly controlled parts in the assembling process.


The np chart
The np Chart

  • Standard given:

    UCL = np0 +3np0 (1-p0)

    LCL = np0 - 3np0 (1-p0)

  • No standard given:

    UCL = x + 3  x (1- x/n)

    LCL = x - 3  x (1- x/n)


The c chart
The c Chart

  • Standard given

    UCL = c0 + 3 c0

    LCL = c0 - 3 c0

  • No standard given

    UCL = c+ 3c

    LCL = c- 3c


The u chart variable unit size
The u Chart(variable unit size)

u = c/k k is the number of inspection

units in each sample

UCL = u0 + 3 u0/k

LCL = u0 - 3 u0/k