Control Limit and the relation between CL with hypothesis test

Control Limits

In SPC the control limit consists of Upper Control Limit (UCL) and the Lower Control Limit (LCL) with a conventional distance of three standard deviations (±3sigma) from the centerline.

The control limits are chosen so that if the process is in control, nearly all the points will fall between them. As long as the new point plots are within the control limits, the process is assumed to be in control, and no action is necessary. In control process refer to “No Special cause effect exist in the process”, the variation only due to common cause effect .In minitab , Nelson test are used to ensure whether the process is in control or not in control .

When we begin to create a control chart, we must wait to have at least 30 initial points to calculate the control limits, which are called trial control limits. They allow us to determine whether the process was in control when the 30 initial samples were selected. If all points fall inside the control limits and no systematic behavior is evident, then we conclude that the process was in control in the past, and the trial control limits are suitable for controlling current and future production. In minitab to maintain the trial control limit for future monitoring , we need to key in the mean and standard deviation resulted from the 30 initial data , in parameter tab .

Generally, the effective use of any control chart will require periodic revision of the control limits and center lines. Some practitioners set regular periodic reviews and revisions of control chart limits, such as every week, every month, or every 25, 50 or, 100 samples.

Let’s see the reason why ±3sigma is used to establish the control limit in SPC .