Control Charts for Measurements with Varying Sample Sizes

Article

Burr, Irving W.   (1969, ASQC)   Purdue University, Lafayette, IN

Journal of Quality Technology    Vol. 1    No. 3
QICID: 5023    July 1969    pp. 163-167
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Article Abstract

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.

Keywords

Standard deviation,Range,Mean,Control charts,Control limits


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