and R Charts With Variable Sample Size and Sampling Intervals

Recent studies show that the x and R charts with variable sample size and with variable sampling intervals detects process shifts faster than the conventional x and R charts. The paper considers the median and range charts with variable sample size and sampling intervals based on Costa model of x Chart. It is assumed that the amount of time the process remains in control has exponential distribution. The proposed charts are compared with the conventional charts, the variable sample size charts and the variable sampling interval charts in terms of the average time to signal an off target process.