SPC Training and Decision Making with OC Curves


Burns, Clarence R.   (1992, ASQC)   Fisher Controls Int., Inc., Marshalltown, IA 50158

Annual Quality Congress, Nashville TN    Vol. 46    No. 0
QICID: 9869    May 1992    pp. 513-517
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Article Abstract

When deciding what methods to use for statistical process control charts (precontrol vs. X-2 bar or R charts), the author recommends designing Operating Characteristics (OC) charts. OC curves picture the behavior of the SPC charts in the same way that SPC charts picture the behavior of the process.

The author uses two kinds of OC curves: Type % (which shows percent out of tolerance) and Type delta (which shows sigma shift). He constructed the sample charts using the detection criteria of one point outside the control limits. Use Type % to evaluate acceptance plan sampling. Use Type delta to evaluate SPC charting.

Type % OC curves can demonstrate if an SPC chart is missing sigma shifts, by means of assuming a normal curve distribution. It compares this to information on where the specification limits are in relation to the curve, which provides percent defective. This curve can also show the effects of changing the sample size or Cpk.

Since OC Curves provide an easy to understand method of illustrating the importance of prevention vs. inspection. Using OC curves provides an important decision-making aide and training tool.


Operating Characteristic (OC) curve,Precontrol,Process improvement,R (range) chart,Sample size,Statistical process control (SPC),X-bar control charts

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