Answer:
First, we need to calculate the control limits for the X-chart. Since the sample size is 4, the standard deviation of the sample mean is:
σ/√n = 2/√4 = 1
The 3σ control limits for the X-chart are:
Upper Control Limit (UCL) = 8 + 3(1) = 11
Lower Control Limit (LCL) = 8 - 3(1) = 5
Next, we need to find the probability that a point falls outside the control limits when the process mean shifts to 9. This can be calculated using the Gaussian distribution:
P(X < 5 or X > 11) = P(Z < (5-9)/2) + P(Z > (11-9)/2) = 0.00135 + 0.00135 = 0.0027
where Z is the standard normal distribution.
Therefore, the average run length (ARL) is:
ARL = 1/p = 1/0.0027 = 370.37
So on average, it will take 370.37 samples before the process mean shift is detected on an X-chart.