Which of the following statements about in-control and out-of-control processes are true?
I. The mean chart and range chart always show that processes are in control.
II. A mean chart can be in control,while a range chart can be out of control.
III. A mean chart can be out of control,while a range chart can be in control.
A)I only
B)II only
C)II and III
D)I and III

Question

Which of the following statements about in-control and out-of-control processes are true? 
I. The mean chart and range chart always show that processes are in control.
II. A mean chart can be in control,while a range chart can be out of control.
III. A mean chart can be out of control,while a range chart can be in control.
A)I only
B)II only
C)II and III
D)I and III

Quizplus · Accepted Answer

The mean and range charts are used to monitor the stability of a process. It is possible for a process to be out of control even if one chart shows it to be in control. This is because each chart monitors different aspects of the process. The mean chart monitors the central tendency of the process, while the range chart monitors the variability of the process. It is possible for the mean to be in control while the variability is increasing, causing the range chart to show out-of-control signals. Similarly, the mean can be out of control while the variability is stable, causing the range chart to show in-control signals. Therefore, statement II and III are true, and statement I is false.

Which of the Following Statements About In-Control and Out-Of-Control Processes