An X and EWMA Chart for Individual Observations

Abstract

While there is much experience in monitoring processes with individual observations it is not clear which procedure is best. We compare individual observation control procedures that are combinations of the moving range (MR), exponentially weighted moving average (EWMA), and X charts, with and without run rules. We estimate the average run length (ARL) to false alarms and to detection of shifts in the process mean and standard deviation using simulation. Our recommendation is the X and EWMA chart, a practical method in that both charts are plotted in a single, easily interpreted graph. It detects large and small shifts in the process mean and large shifts in the process standard deviation.

• Control Charts
• Exponentially Weighted Moving Average Control Chart
• Individual Measurements
• Moving Range Control Charts
• X Control Charts

Additional information

Notes on contributors

Susan L. Albin

Dr. Albin is a Professor in the Department of Industrial Engineering. She is a Senior Member of ASQC.

Lan Kang

Ms. Kang is a Doctoral Student in the Department of Industrial Engineering.

Gerald Shea

Dr. Shea is an Engineering Associate. He is a Member of ASQC.