Control Charts for Attributes

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# Control Charts for Attributes - PowerPoint PPT Presentation

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

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Presentation Transcript
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 +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
• 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)

u = c/k k is the number of inspection

units in each sample

UCL = u0 + 3 u0/k

LCL = u0 - 3 u0/k