Complete the following steps to interpret a T^{2} chart. Key output includes the T^{2} chart and test results.

The T^{2} chart plots the T^{2} for each subgroup to measure whether the process locations of several related variables are simultaneously in control. The center line is the median of the theoretical distribution of T^{2} statistics. The upper control limit is based on the number of samples, the size of each sample, and the number of variables.

Red points indicate subgroups that are above the upper control limit and not in control. One disadvantage to multivariate charts is that the scale is unrelated to the scale of any of the variables, and out-of-control signals do not reveal which variable (or combination of variables) caused the signal.

Investigate any subgroups that are above the upper control limit. The output shows which points are above the upper control limit, as shown here.

Test Results for T² Chart of Stay, Satisfaction
Point Variable P-Value
Greater Than UCL 2 Stay 0.0030
Satisfaction 0.0067
18 Stay 0.0010
Satisfaction 0.0002
19 Satisfaction 0.0000
* WARNING * If graph is updated with new data, the results above may no longer
be correct.