Four samples of three observations each have been taken, with actual measurements ...

     Four samples of three observations each have been taken, with   actual measurements (in centimeters) shown below. Construct three sigma mean   and range (X-bar and R) charts, and determine if corrective action is needed.   Sample 1 12.3 12.2 12.1   Sample 2 11.9 12.2 12.2   Sample 3 12 12.2 11.8   Sample 4 12.1 11.8 11.8