For nonnormal capability sixpack, Minitab displays different control charts depending on the subgroup size.
If the subgroup size is 1, Minitab displays an I-MR chart. To calculate the control limits in this case, Minitab estimates the probability density function from the data and uses this function to calculate the 0.135th and 99.865th percentiles. The values that correspond to these percentiles represent the lower and upper control limits.
If the subgroup size is greater than 1, Minitab displays an Xbar chart with an R or S chart.