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Hi all,
I am running a process where we take measurements on 5 pcs of product every 2 hours. So far, I have 221 sets of measurements (each set has 5 measurements).
I am planning to implement X-bar R chart on this process. I calculated the control chart limits using these 2 different methods:
Method 1: Use the control limit formula
Control Limits = (Average of X-bars) +/- (A2) x (Average of Rs)
= 2.376 +/- (0.577) x (0.097)
Result, UCL = 2.432, LCL = 2.320
Method 2: Use Excel's built-in formula STDEV()
Control limits = (Average of X-bars) +/- 3 x STDEV(all the individual Xs)
= 2.376 +/- 3 x (0.101)
Result, UCL = 2.679, LCL = 2.073
I find the difference between the 2 methods to be too much. Does anyone know what might be the problem?
Thanks.
I am running a process where we take measurements on 5 pcs of product every 2 hours. So far, I have 221 sets of measurements (each set has 5 measurements).
I am planning to implement X-bar R chart on this process. I calculated the control chart limits using these 2 different methods:
Method 1: Use the control limit formula
Control Limits = (Average of X-bars) +/- (A2) x (Average of Rs)
= 2.376 +/- (0.577) x (0.097)
Result, UCL = 2.432, LCL = 2.320
Method 2: Use Excel's built-in formula STDEV()
Control limits = (Average of X-bars) +/- 3 x STDEV(all the individual Xs)
= 2.376 +/- 3 x (0.101)
Result, UCL = 2.679, LCL = 2.073
I find the difference between the 2 methods to be too much. Does anyone know what might be the problem?
Thanks.