Year

Mean

Mann-Whitney U

Wilcoxon

Z-Statistics

Asymptotic

Significant

(2 Tailed)

Sen’s Estimator

Number of Samples

1971-1980

1981-1990

12.9

8.1

26

81

−1.814

0.070

−0.775

10

10

1991-2000

2001-2010

11.4

9.6

41

96

−0.680

0.496

−0.094

10

10