Comparing the effects of skewed distributions on the S-chart and S2 -EWMA chart based on the median run length criterion

Ker Hsin Ong, Sin Yin Teh, Michael Boon Chong Khoo, Wei Lin Teoh, Keng Lin Soh

Research output: Contribution to journalArticlepeer-review

1 Citation (Scopus)


The S2-EWMA (called the S square exponentially weighted moving average) control chart is effective in detecting small and moderate process variance shifts. Previously, the chart was designed based on the assumption that the distribution of the quality characteristic is normally distributed. This study designs the S2-EWMA control chart for skewed distributions. The skewed distributions considered in this paper are the lognormal and gamma distributions. The performance of the S2-EWMA control chart is compared with that of the traditional Shewhart S-chart, in terms of median run length (MRL), based on simulation using the Statistical Analysis System (SAS). The results show that regardless of the type of skewed distributions, sample size and skewness level, γ, in most of the cases, the S2-EWMA chart outperforms the S-chart. Moreover, the findings reveal that the MRL performances of the S-chart and S2-EWMA chart are significantly influenced by skewed distributions.

Original languageEnglish
Pages (from-to)53-58
Number of pages6
JournalJurnal Teknologi
Issue number6-6
Publication statusPublished - Jun 2016


  • Median run length
  • Process variability
  • S-chart
  • S-EWMA control chart
  • Skewed distributions

ASJC Scopus subject areas

  • Engineering(all)


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