Variable Sampling Interval Control Charts with Sampling at Fixed Times

This paper proposes median and range variable sampling interval control charts with sampling at fixed times based on Reynolds model of x control chart, working out the average time to signal for \widetildex and R charts and combined \widetildex and R charts under variable sampling intervals with sampling at fixed times. In the meantime, the author compares these charts with the fixed sampling intervals \widetildex and R charts.