Making the system stable
Step Three

At first glance, a control chart  resembles a time series graph. In a time series graph, e.g. monthly sales, we plot months of the year along the horizontal axis, and the number of products sold along the vertical axis. However, this graph does not give us sufficient information about the behavior of a process, whether it is in or out of control, predictable or not. It is not sufficient to make comparisons between single values on such a chart.
The control chart, which in every respect is a process behavior chart, instead puts this information into a context by adding three horizontal lines. The central line acts as a reference against which we identify  trends. The other two lines are control limits – the upper and lower control limits, or natural process limits. These are calculated with the help of  coefficients  using the average values from a time series graph and moving ranges, i.e. the differences between the individual values of the time series.

 see a control chart of a stable process 

 see a control chart of a process out of control