C Control Chart (Number of Defects) Online
Online c chart: monitor the number of defects per inspection unit (constant sample size), with control limits of c̄ ± 3√c̄ based on the Poisson distribution and detection of out-of-control patterns.
How do I choose between a c chart and a u chart?
Use a c chart when the inspection unit size is constant (e.g., one circuit board each); use a u chart when the inspection unit size varies (e.g., fabric rolls of different areas).