Which of the following statements is NOT true with regard to X-Bar control charts?

OPTIONS

• The width of the limits is usually set at plus or minus 3 standard deviations of the process.
• If the process is in control, 2700 points per million will fall outside the control limits.
• If the process is in control, half the points will fall below the center line.
• The width of the limits is inversely proportional to the square root of the sample size.

ANSWER

The width of the limits is usually set at plus or minus 3 standard deviations of the process.

EXPLANATION

The control limits are usually set at plus or minus 3 standard deviations of the sampling distribution used, not the process. If the sample size is 1, these are the same. In general the standard deviation of the sampling distribution is equal to the standard deviation of the individuals divided by the square root of the sample size.