Use the Z chart to assess whether the process center is in control. The Z chart consists of:

·    Plotted points, which are standardized values.

·    Center line (green), which the estimate of the process average across runs. The center line is always located at zero because the data is standardized. Standardized data come from a population with m = 0 and s = 1.

·    Control limits (red), which are located 3 s (standard deviations) above and below the center line and provide a visual means for assessing when the process is out of control. The lower and upper control limits are always -3 and +3, respectively, because the data is standardized. Standardized data comes from a population with s = 1 and m = 0.

·    Dashed vertical lines, which separate one run from the next.

You should examine the Z chart to see if it is in control.

·    Determine whether or not any points are located above or below the upper and lower control limits.

·    For the stamp data, no points are located beyond the control limits.

·    Compare the pattern of data points across runs. Do the points fall within the control limits in a random manner? Does one run have larger standardized values than other runs?

For the stamp data, the standardized values for Run B seem to vary more than those for Run A or Run C. Thus, there may be special causes influencing the process. At this point, you would want to try to identify and correct the factors contributing to the special-cause variation. Until these causes are eliminated, the process cannot achieve a state of statistical control.