
Quality in Production—Process Control II 307
(or 0�2 of rejecting) a process that has moved through a 1σ distance� Only
when the process shift is greater than 1�5σ does this X-chart have any rea-
sonable probability (>0�50) of rejection� Also, notice that when the sample
size increases, the power of the control chart, (1 − β), increases for detecting
a given amount of change in the process mean� When n = 16, the power to
detect a change of lσ distance in the process mean becomes 0�84�
We can see from the above OC curves that the conventional X-chart with
n = 4 is not quite sensitive to changes in the process mean unless the change
in the mean ...