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Transformed p Chart for Variable Sample Size

Summary: The conventional p chart requires use of variable control limits or the use of approximations if the sample size varies. An alternative control chart variable, obtained by a transformation of the fraction defective, has been proposed to avoid some of these problems. The alternative chart has a standard center line and, except for small samples, constant control limits. The conditions for nonconstant control limits and their positions are evaluated. In borderline cases, the alternative chart shows the extent by which a point is in or out of control more clearly than the conventional chart. The two kinds of charts are compared by means of an example.

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  • Topics: Sampling
  • Keywords: Control charts,Sample size
  • Author: Soffer, Stephen B.
  • Journal: Journal of Quality Technology