The Estimation of Sigma for an X Chart: /d2 or S/c4?

Article

Cryer, Jonathan; Ryan, Thomas   (1990, ASQC)   University of Iowa

Journal of Quality Technology    Vol. 22    No. 3
QICID: 11278    July 1990    pp. 187-192
List $10.00
Member $5.00

FOR A LIMITED TIME, ACCESS TO THIS CONTENT IS FREE!
You will need to be signed in.
New to ASQ? Register here.

Article Abstract

The standard approach to estimating sigma for an individual observations control chart is to use moving ranges. We show that, for independent, normally distributed observations, this approach is very inefficient compared to using the sample standard deviation. Thus, for future process monitoring the estimation of sigma should be based on the sample standard deviation. It is also shown that the moving range approach can provide especially poor results when the observations are correlated over time. This is demonstrated with some real chemical data and it is shown how certain processes that are in control can produce correlated data. Some recommendations are given concerning the estimation of sigma for individual observations control charts.

Keywords

Control charts,Standard deviation,Moving range control charts,Correlation


Browse QIC Articles Chronologically:     Previous Article     Next Article

New Search

Featured advertisers





ASQ is a global community of people passionate about quality, who use the tools, their ideas and expertise to make our world work better. ASQ: The Global Voice of Quality.