Any help on this control chart question?

Q:A production process has been defined as part of an industrial equipment
manufacturing project.The process is intended to produce steel bolts with a
length of 20 cm.The acceptable control limits are 19.955 cm and 20.045 cm.
The measurements made at the end of the process yielded the following
20.033 cm, 19.982 cm, 19,995 cm, 20.006 cm, 19.970 cm, 19.968 cm,
19.963 cm, 19.958 cm, 19.962 cm, 19.979 cm, 19.959 cm.
What should be done?
A. The process is under control.It should not be adjusted.
B. The process should be adjusted.
C. The control limits should be adjusted.
D. The measuring equipment should be recalibrated


Ans is . B: The process should be adjusted.

Can pls soemone explain why?bcoz all the results are within control limit.So i was choosing option A.

Hi Sunita,

Plot the control chart. As you can see you have been given 3 values , LCL(19.955cm), HCL(20.045cm) and the mean(20cm).. If you plot the different values mentioned in the control chart, you can very well see there are more than 7 points on one side of the mean which indicates its a special case variation and the process needs to be investigated/out of control.



7 consecutive


The control chart is used to determine whether the process is within acceptable limit (Upper and lower limit) with respect to mean. And if 7 consecutive points of reading on either side then the process is said to out of control and should be immediately stop. In your case the same thing is happening,

Hence  Option B is correct