Vssi charts xharts with sampling at fixed times

Abstract

A standard chart for controlling the process mean takes samples of size n at specified, equally-spaced, fixed-time points. This article proposes a modification of the standard chart that allows one to take additional samples, bigger than n, between these fixed times. The additional samplesare taken from the process when there is evidence that the process mean moved from target. Following the notation proposed by Reynolds (1996a) and Costa (1997) we shortly call the proposed chartas VSSIFT chart where VSSIFT means variable sample size and sampling intervals with fixed times.The chart with the VSSIFT feature is easier to be administered than a standard VSSI chart thatis not constrained to sample at the specified fixed times. The performances of the charts in detecting process mean shifts are comparable.

Keywords:

• average time to signal
• X control charts
• statistical process control
• variable sample size
• variable sampling interval