An attribute control chart for monitoring the variability of a process

Recently, Wu et al. (2009) proposed the use of an npx chart to monitor a process mean by attribute inspection as an alternative to the use of an X¯ chart. Motivated by the simplicity of this control chart and its good performance, the possibility of using a similar chart, an npS2 control chart, to monitor the variability of a process was explored, and the results are described in this paper. A comparison of the proposed npS2 control chart with the S2 and R control charts is presented, and a numerical example is provided to demonstrate the use of the npS2 control chart.