Control charts show the performance of a process from two points of view. First, they show a snapshot of the process at the moment data is collected. Second, they show the process trend as time progresses. Process trends are important because they help in identifying the out of control status if it actually exists. Also, they help to detect variations outside the normal operational limits and to identify the causes of variations. Above figure shows the generalised representation of control chart.
The UCL and LCL jointly specify the range over which each sample observation can lie. After plotting all the sample observations on the chart, we should look for the pattern of those plots. If any sample observation is outside of these two limits, we can conclude that the process is out of control and definitely requires corrective action.