Title: A new X chart comparable to CUSUM and EWMA charts

Authors: D.R. Prajapati, P.B. Mahapatra

Addresses: Department of Mechanical Engineering, Punjab Engineering College, Chandigarh 160012, India. ' Department of Mechanical Engineering, Punjab Engineering College, Chandigarh 160012, India

Abstract: The most popular and efficient charts to catch small shifts in process mean are CUSUM and EWMA charts. However, they catch the shift on process average if there is a single and sustained shift, and are unable to detect the changes when samples are not taken from same stream. In such situation, the new chart is proposed in the present work and compared with conventional CUSUM and EWMA charts. It is found that both CUSUM and EWMA schemes are ineffective to catch the process shifts in such cases. It can be noted that the new (proposed) chart provides the highest percentage of correct signals compared to CUSUM and EWMA schemes for all process shifts. It is also clear from the study that for all the process shifts, the EWMA scheme is most ineffective out of the three schemes. The average run length (ARLs) of proposed scheme is much lower than both CUSUM and EWMA schemes for all the shifts in the process average.

Keywords: average run length; ARL; chi-squares; x-bar charts; CUSUM; EWMA; correct signalling; control charts; SPC; statistical process control.

DOI: 10.1504/IJPQM.2009.021873

International Journal of Productivity and Quality Management, 2009 Vol.4 No.1, pp.103 - 128

Available online: 07 Dec 2008 *

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