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## PowerPoint Slideshow about ' Control Charts for Attributes' - bunme

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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

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

- 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

- Standard given:

UCL = np0 +3np0 (1-p0)

LCL = np0 - 3np0 (1-p0)

- No standard given:

UCL = x + 3 x (1- x/n)

LCL = x - 3 x (1- x/n)

The c Chart

- Standard given

UCL = c0 + 3 c0

LCL = c0 - 3 c0

- No standard given

UCL = c+ 3c

LCL = c- 3c

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

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