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T20-04 C Chart Control Limit Calculations
Purpose
Allows the analyst to calculate the defectives per unit
"C-Chart" 3-sigma control limits.
Inputs
Average Defective Per Unit (Chat)
Outputs
C (Defects Per Unit) Control Chart LCL & UCL
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C-Chart (Defects Per Unit)
The C-Chart is used to monitor the number of defects per unit (e.g.
automobiles, hotel rooms, typed pages, rolls of carpet).
The C Chart Centerline, UCL and LCL are calculated by the following
formula.
UCL = c + 3 c Standard Error
LCL = c - 3 c where
c = process average number of defects per unit
In the event c is unknown, it can be estimated by sampling and
computing the average defects observed. In this case the average can
.
be substituted in the formula for c
Also, note that since the formula is an approximation the LCL can be
negative. In these cases the LCL is set to 0.
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Example
Number
Defects
3
2
4
5
1
2
4
1
2
1
3
4
2
4
2
1
3
1
Rolls of coiled wire are monitored using a c-chart. Eighteen
rolls of wire have been examined and the number of defects
per roll has been recorded in the data provided here. The
average defective per roll (Chat) can now be calculated.
This can be done by using the Descriptive Statistics
Template or more quickly by using the EXCEL built in
AVERAGE function on this set of numbers.
Calculate the Control Limits for the C Control Chart.

c  2.5
Average Defects per Roll
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Input the Average Defective Per Unit (Chat) in
the light green cells.
The LCL, UCL are
automatically calculated
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