Given the following process control data for a normally distributed quality variable (three samples of size four each): If the process is known to have a mean of 15 and a standard deviation of 3 , what are the three sigma upper and lower control limits for an x bar chart? Given the following process control data for a normally distributed quality variable (three samples of size four each): If the process is known to have a mean of 15 and a standard deviation of 3 , what are the three sigma upper and lower control limits for an x bar chart