1970 or earlier

72.14

1971

64.15

1972

47.16

1973

34.53

1974

32.22

1975

30.58

1976

28.29

1977

26.38

1978

22.65

1979

17.45

1980

14.39

1981

13.61

1982

13.34

1983

11.95

1984

10.86

1985

9.97

1986

8.70

1987

7.38

1988

5.71

1989

4.80

1990

4.89

1991

5.00

1992

5.25

1993

5.36

1994

5.22

1995

5.21

1996

5.11

1997

4.63

1998

4.17

1999

3.72

2000

3.10

2001

2.72

2002

2.25

2003

1.92

2004

1.69

2005

1.61

2006

1.51

2007

1.37

2008

1.43

2009

1.59

2010

1.51

2011

1.52

2012

1.49

2013

1.43

2014

1.28

2015

1.19

2016

1.10

2017

1.04

2018

1.04

2019

1.05

2020

1.04