Charts | u Chart
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p Chart pn Chart u Chart c Chart X-Bar R X-Bar s I-MR / X-MR Out-of-Control
A u chart is used when we’re looking a the number of defects and the sample size is variable. For instance, you may check the number of pin-holes in enamel wire or the unevenness of a material. This is where u charts are useful.
Control Limits
Since the subgroup size is NOT constant, so the upper and lower control limits (UCL & LCL) are NOT constant, either. So, for every sample, there will be a separate upper and lower control limit. This results in having a narrower control limit the larger your sample size is. Remember, u charts represent a Poisson distribution. Therefore, the Poisson calculations apply.
- Control Line (CL) =
u = total defects for all subgroups (c)/total units for all subgroups (n) = Σc/Σn - Upper Control Limit (UCL) =
u + 3√ u⁄n - Lower Control Limit (LCL) =
u - 3√ u⁄n