| Year | Value | | 2022 | 7.8326912423332 | | 2021 | 2.5183710961421 | | 2020 | 0.63854495677671 | | 2019 | 1.625077254692 | | 2018 | 1.7947118515184 | | 2017 | 1.7626493189224 | | 2016 | 0.62504992308451 | | 2015 | 0.41004049800847 | | 2014 | 0.94141454006696 | | 2013 | 1.4234394820853 | | 2012 | 2.4856756217702 | | 2011 | 3.3298532335065 | | 2010 | 1.9962123674589 | | 2009 | 0.83926224679389 | | 2008 | 4.1527970811029 | | 2007 | 2.3276112889148 | | 2006 | 2.4579244419586 | | 2005 | 2.3753195897788 | | 2004 | 2.0792446463609 | | 2003 | 2.0216854285799 | | 2002 | 1.9987492114935 | | 2001 | 2.6569109392932 | | 2000 | 2.5798761121861 | | 1999 | 1.6117611762866 | | 1998 | 1.8343299990841 | | 1997 | 2.2011431351088 | | 1996 | 2.6153846153846 | | 1995 | 3.7549441367755 | | 1994 | 2.9534093676108 | | 1993 | 4.1448195418049 | | 1992 | 4.0370302077874 | | 1991 | 5.62586408593 | | 1990 | 6.098444148102 | | 1989 | 5.7159915513563 | | 1988 | 4.6862018375304 | | 1987 | 4.0665111594411 | | 1986 | 3.1590972945193 | | 1985 | 4.773742723401 | | 1984 | 6.1947637954598 | | 1983 | 7.3104857966842 | | 1982 | 9.4709893205241 | | 1981 | 11.767387434418 | | 1980 | 11.949971662614 | | 1979 | 9.0492095942541 | | 1978 | 8.0832988258853 | | 1977 | 9.2991971917537 | | 1976 | 9.267754575769 | | 1975 | 11.687575365504 | | 1974 | 13.16360261208 | | 1973 | 8.094439616028 | | 1972 | 6.0630030971819 | | 1971 | 6.1381074168798 | | 1970 | 4.9683198732933 | | 1969 | 3.2786885245902 | | 1968 | 3.7207932801914 | | 1967 | 3.6348751533576 | | 1966 | 3.8446565434989 | | 1965 | 3.6664828026259 | | 1964 | 3.3346246289278 | | 1963 | 2.8740314034922 | | 1962 | 3.5198845401881 | | 1961 | 2.0771383313786 | | 1960 | 1.536612342952 |
|