charts assess whether the process center is in control. The chart consists of the following:

·    Plotted points, each of which represents the average of measurements in that subgroup.

·    Center line (green), which is the estimate of the process average (average of all subgroup averages).

·    Control limits (red), located 3 s above and below the center line, provide a visual means for assessing when the process is out of control. The control limits, which represent the amount of variation expected for the subgroup averages, are calculated using the variation within-subgroups.

Minitab conducts up to eight tests for special causes for the chart, which detect points beyond the control limits and specific patterns in the data. Points that fail are marked with a red symbol and the number of the failed test. Complete results are available in the Session window. A failed point indicates that there is a nonrandom pattern in the data that may be the result of special-cause variation. These points should be investigated.

The R chart must be in control before you can interpret the chart. If the R chart is not in control then the control limits for the chart will be inaccurate and may inappropriately signal an out-of-control condition on the chart.

For the camshaft data, the R chart is in control, therefore it is appropriate to examine the chart. The chart for the camshaft data can be summarized as follows:

·    The lower and upper control limits are 598.503 and 601.641, respectively. Therefore, the subgroup averages are expected to fall between 598.503 and 601.641. The center line (estimate of process average) is 600.072.

·    One subgroup, located below the lower control limit 598.503 , failed Test 1 because it is more than 3 s below the center line. Test 1 is the strongest indicator of an out-of-control process.

·    Four subgroups failed Test 6, which looks for four out of five subgroups more than one s from, and on the same side of, the center line

·    One subgroup failed Test 5, which looks for two out of the three subgroups more than two s from, and on the same side of, the center line.

These test results indicate that the process average is unstable and the process is out of control, possibly due to the presence of special causes. Next, you would try to identify and correct the factors contributing to this special-cause variation. Until these causes are eliminated, the process cannot achieve a state of statistical control.