Individuals Charts and Additional Tests for Changes in Spread

Some authors recommend the use of an additional test for detecting increases in the spread, when using a control chart for individual observations. We examine this recommendation both in a practical situation and theoretically. Both studies show that the additional test gives somewhat more power for detecting a 25% increase of the process variation. For nearly all other deviations from the in-control state the test is more likely to cause confusion. From a practical viewpoint we therefore advise against its use.

[1]  Douglas C. Montgomery,et al.  Introduction to Statistical Quality Control , 1986 .

[2]  E. S. Page.  A test for a change in a parameter occurring at an unknown point , 1955 .

[3]  E. S. Page.  CONTROL CHARTS WITH WARNING LINES , 1955 .

[4]  S. Albin,et al.  An X and EWMA chart for individual observations , 1997 .

[5]  Donald J. Wheeler,et al.  Understanding Statistical Process Control , 1986 .

[6]  Lloyd S. Nelson,et al.  Column: Technical Aids: Monitoring Reduction in Variation with a Range Chart , 1990 .

[7]  Jakob Edo Wieringa.  Statistical process control for serially correlated data , 1999 .

[8]  Thomas P. Ryan,et al.  Statistical methods for quality improvement , 1989 .

[9]  D. Hawkins.  A Cusum for a Scale Parameter , 1981 .

[10]  G. Barrie Wetherill,et al.  Statistical Process Control , 1991 .

[11]  S. R. Adke,et al.  A Supplementary Test Based on the Control Chart for Individuals , 1997 .

[12]  G. B. Wetherill,et al.  Quality Control and Industrial Statistics , 1975 .

[13]  Raid W. Amin,et al.  A Note on Individual and Moving Range Control Charts , 1998 .

[14]  P SandorfJ,et al.  The OCAP: Predetermined Responses to Out-of-Control Conditions. , 1993 .

[15]  R. Does,et al.  Shewhart-Type Charts in Nonstandard Situations , 1995 .

[16]  Charles W. Champ,et al.  Design Strategies for Individuals and Moving Range Control Charts , 1994 .

[17]  Begnaud Francis Hildebrand,et al.  Introduction to numerical analysis: 2nd edition , 1987 .

[18]  Joseph J. Pignatiello,et al.  Monitoring Process Dispersion without Subgrouping , 2000 .

[19]  The LR-Chart: An Alternative to the MR-Chart , 2003 .

[20]  Ronald J. M. M. Does,et al.  Shewhart-Type Control Charts for Individual Observations , 1993 .

[21]  Robert N. Rodriguez,et al.  Computer Programs: A Program for the Computation of ARL for Combined Individual Measurement and Moving Range Charts , 1987 .

[22]  Stephen V. Crowder.  Computation of ARL for Combined Individual Measurement and Moving Range Charts , 1987 .

[23]  D. Coleman.  Statistical Process Control—Theory and Practice , 1993 .

[24]  Lloyd S. Nelson,et al.  Control Charts for Individual Measurements , 1982 .

[25]  Charles W. Champ,et al.  Exact results for shewhart control charts with supplementary runs rules , 1987 .