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SPC Q Charts for a Poisson Parameter Lambda: Short or Long Runs

Summary: Approximately normalized control charts, called Q Charts, have been given by the author in other work for charging variables and for a binomial attributes chart. We consider here the corresponding transformations for Poisson distributions, and give the Q charts technology for controlling a Poissonparameter lambda. Charts are given for the two cases when the Poisson parameter lambda is known and when it is unknown. The chart for the case when lambda is unknown is useful both for short runs and for charting processes at start up before enough data are available to assume that lambda is essentially known. The chart for the case when the parameter is assumed known gives probabilities of exceeding the control limits that are better approximated by the nominal normal distribution probabilities than the classical c or u charts. We give some tabular results to compare the goodness of the normal approximation to the probabilities for these charts with the c or u charts, and also with a chart made using a square root normalizing transformation.

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  • Topics:
  • Keywords: C chart (count chart),Start up process(es),Short runs,Nonconformities,u chart (count per unit chart),Job shop
  • Author: Quesenberry, Charles
  • Journal: Journal of Quality Technology