Charts | c Chart
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A c chart is used when we’re looking a the number of defects and the sample size is constant. For instance, you may be tallying the number of cracks in products that are otherwise identical. This is where c charts are useful.
Control Limits
Since the subgroup size is constant, so the upper and lower control limits (UCL & LCL) are also constant. Remember, c charts represent a Poisson distribution. Therefore, the Poisson calculations apply.
- Control Line (CL) =
c = total defects for all subgroups (c)/total units for all subgroups (n) = Σc/Σn - Upper Control Limit (UCL) =
c + 3√ c - Lower Control Limit (LCL) =
c - 3√ c