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Calculating the (Almost) Exact Control Limits for a C-Chart

Summary: [This abstract is based on the author's abstract.]With the addition of a small constant, a power transformation using an exponent of 2/3 for Poisson-distributed data can achieve symmetry for improved statistical process control applications. Two equations are proposed for calculating the lower control limit (LCL) and the upper control limit (UCL) for Poison-type data. Agreement between the exact LCL and UCL is excellent. The square root transformation that stabilizes the variance produces a negatively skewed distribution and tends to give false SPC signals.

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  • Topics: Statistical Process Control (SPC)
  • Keywords: Control charts,Control limits,Poisson distribution,Statistical process control (SPC),Transformation
  • Author: Kittlitz, Rudolf G. Jr.
  • Journal: Quality Engineering