# Quality Control Line Chart in Excel

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Control Line Chart

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:

1. Upper Control Limit (UCL). (Mean + K*S)
2. Central Line (CL). (Mean)
3. Lower Control Limit (LCL). ((Mean – k*S)

How to Create Control Line Chart in Excel

1. 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.
2. Calculate the overall mean of the data points from X-Bar, Overall mean will be the central line in the control chart.
3. Calculate the standard deviation(S) using formula “STDEV”.
4. Calculate the UCL and LCL using the following formulae:
5. UCL = CL + K*S
6. LCL = CL- K*S
7. 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.
8. 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:

1. Any point falls above or below the 3-sigma limit (Red Line).
2. 8 consecutive points fall on one side of the centerline.
3. Two consecutive points falls above the +2 Sigma limit (green Line).
4. 10 consecutive points falling below the centerline.
5. 4 of 5 consecutive points falling below the -2Sigma limit (Green line).