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Control Charts for Measurements with Varying Sample Sizes

Summary: When sampling sizes vary, analysis by control charts must be modified from that for equal sample sizes. A simple weighted average of the X-bars, with weights n, is taken for X-double bar in general. Usually when n's are small, ranges R are used. Then to estimate sigma for the process (if variabilities are indeed homogeneous) an unweighted average of the unbiased estimates, R/d2, has commonly been used. This paper provides two systems of simple weights from which better results are obtainable. Similarly, weights for sample standard deviation are also given.

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  • Topics:
  • Keywords: Standard deviation,Range,Mean,Control charts,Control limits
  • Author: Burr, Irving W.
  • Journal: Journal of Quality Technology