Combine Median and Range Charts with Variable Sample Sizes and Sampling Intervals

This paper extends previous studies for processes that are monitored by the combine x and R charts with variable sample sizes and sampling intervals based on Costa model of joint x and R charts, it is assumed that the amount of time the process remains in control has exponential distribution. Calculating the average time to signal for the combine x and R charts under the variable sample size and sampling interval. The proposed charts are compared with the conventional charts, the variable sample size charts and the variable sampling interval charts.