计算出的CPK有负值是怎么回事？

我用minitab软件计算CPK时，结果出现负值。
数据如下：
96.7
97.4
95.3
97.4
99.3
95.9
93.8
97.2
94.5
101.4
95.4
98.3
98.6
95.8
95.9
96.4
100.7
96.6
95.7
96.4
97.5
95.8
96.2
97.0
99.7
96.6
98.3
99.6
96.9
97.6
99.9
98.5
99.0
94.5
100.6
96.5
95.8
99.6
97.7
97.0
96.5
96.3
99.4
95.5
96.6
97.9
98.5
99.8
102.7
95.2
96.7
97.2
求助高手解答，谢了！