the maximum allowable variation of a process characteristic as a result of common causes alone. Calculated from process data and usually presented as a line (or lines) on a control chart. Not to be confused with engineering specification limits. [EIA 557

a statistical concept that indicates regions where a random variable's values are considered to be statistically unlikely (unusual) for a given process. Often these limits are 3 standard deviations above and below the mean. When an observation is further from the mean than the control limit, one infers that a special cause of variation exists and that remedial action to control the process is appropriate.

Natural process limits, determined from historical data of how the process will run if undisturbed. The control limits are at the historical mean or target +/- 3 x the historical standard deviation.

In statistical quality control, the upper and lower values of a measured quantity that establish the range of acceptability; if any individual measurement falls outside this range, the part involved is rejected and if the sample average for the same measurement falls outside the range, the entire lot is rejected.

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