 The main advantage of distribution-free control charts is its in-control robustness, in the sense that, irrespective of the nature of the underlying process distributions, the properties of these control charts remain same when the process is smoothly operating without presence of any assignable cause.Early evidence of research on nonparametric control charts may be found in 1981 when P.K.Bhattacharya and D. Frierson introduced a nonparametric control chart for detecting small disorders.