Kane [7] carried out a study on capability indices and examined the uses of capability indices along with their estimation procedure.
When properly applied, statistical tools are an effective way for improving process quality. Costa [4] also studied the performance of X chart with variable sample size and variable sampling intervals.
He compared the performance of his charts with Shewhart X charts and noticed that the charts proposed by him outperforms Shewhart‟s X chart with a large margin.