Control limits define the area three standard deviations on either side of the centerline, or mean, of data plotted on a control chart. Do not confuse control limits with specification limits. Control limits reflect the expected variation in the data and are based on the distribution of the data points. Minitab™ calculates control limits using collected data. Specification limits are established based on customer or regulatory requirements. Specification limits change only if the customer or regulatory body so requests.