L is the control limts coefficient that may be fixed at a pre-specified value of average run length (ARL), denoted by ARL0 for an in-control process. Run length (RL) is a random variable which measures the variety of ponits plotted on the chart till and out-of-control condition is signaled. ARL is the expected number of points potted on the control chart until an out of signal indication and denoted by ARL.