An economic-statistical design of the joint (X)over-bar and S control charts with double sampling and variable sampling intervals

A combination of double sampling and variable sampling interval (DSVSI) charts can increase the efficiency of signaling small shifts. This study proposes an economic-statistical model for designing joint DSVSI X and s charts. The proposed approach uses the economic model developed by Duncan in 1956. Research results indicate that the economic-statistical design of the joint DSVSI X and s charts outperformed traditional economic design approach in detection abilities of process shifts; variable cost (a(2)), expected costs resulting from defective products when the process was in control (C-0), and the expected number of assignable causes (lambda) have the major influence on cost E(C'); deviations (delta, gamma)significantly influenced cost E(C'); and statistical limitations (AATS(IC)*; AATS(OOC)*) do not substantially affect cost E(C').