Year

Mean

Mann-Whitney U

Wilcoxon

Z-Statistics

Asymptotic

Significant

(2 Tailed)

Sen’s Estimator

Number of Samples

1971-1980

1981-1990

11.3

9.7

42

97

−0.605

−0.545

0.146

10

10

1991-2000

2001-2010

11.6

9.4

39

94

−0.832

−0.406

−0.535

10

10