Control Charts are generally used in manufacturing environment to control, monitor and improve the process. The Control chart shows the value of quality characteristics versus time or versus sample numbers
A control Chart is a graph with three kinds of control lines:
- Upper Control Limit (UCL). (Mean + K*S)
- Central Line (CL). (Mean)
- Lower Control Limit (LCL). ((Mean – k*S)
How to Create Control Line Chart in Excel
- Calculate X-Bar First, Find the mean of each group (sum of observations divide by no. of observations). In our Worksheet, Column G from 12 is X-Bar values.
- Calculate the overall mean of the data points from X-Bar, Overall mean will be the central line in the control chart.
- Calculate the standard deviation(S) using formula “STDEV”.
- Calculate the UCL and LCL using the following formulae:
- UCL = CL + K*S
- LCL = CL- K*S
- In the formula K is the standard deviations above and below the mean, In our template we have used 2 and 3 standard deviations above and below the mean.
- Now create a line chart between X-Bar, UCL, CL and LCL, where measurement would be at y axis.
How to read a Control Line chart:
If any of the below mentioned Point is true graph is out of control:
- Any point falls above or below the 3-sigma limit (Red Line).
- 8 consecutive points fall on one side of the centerline.
- Two consecutive points falls above the +2 Sigma limit (green Line).
- 10 consecutive points falling below the centerline.
- 4 of 5 consecutive points falling below the -2Sigma limit (Green line).