Control limits refer to the wide area of variation that can exist when plotting the actual data that has been charted. The control limits, more specifically, refer to the three standard deviations on either side of the mean (this mean is also known as the centerline), of a normal distribution of data that has been laid out, or plotted, on a control chart. These control limits will typically reflect an expected variation in the data. These expectations can be derived from past experiences, meaning, the deviations that have occurred when conducting similar projects of specific work tasks within then, or they can be derived from nothing more than careful consideration and discussion among the team. Regardless, it is important to utilize these control limits as it better will prepare the control team for possible variations and make them easier to deal with and compensate for. It is important to develop control limits at the outset of a project and/or work task.
This term is defined in the 3rd and the 4th edition of the PMBOK.