| Year | Value | | 2015 | 35.503663832321 | | 2014 | 33.889935902036 | | 2013 | 33.973916812202 | | 2012 | 33.721306380464 | | 2011 | 36.462670547649 | | 2010 | 37.695926642757 | | 2009 | 34.723748713202 | | 2008 | 37.16317450739 | | 2007 | 37.20536770103 | | 2006 | 35.173228421068 | | 2005 | 33.404633424273 | | 2004 | 31.66687890724 | | 2003 | 31.21804415028 | | 2002 | 29.629673575644 | | 2001 | 30.406970361743 | | 2000 | 29.651508636889 | | 1999 | 30.172129547156 | | 1998 | 29.885119851043 | | 1997 | 26.988779391721 | | 1996 | 28.201416885105 | | 1995 | 26.688658954996 | | 1994 | 25.157386420499 | | 1993 | 26.411130692318 | | 1992 | 26.059306597851 | | 1991 | 29.084245042541 | | 1990 | 31.172844835679 | | 1989 | 24.106705214642 | | 1988 | 22.001594529265 | | 1987 | 22.304657016059 | | 1986 | 20.681776659571 | | 1985 | 19.860329812765 | | 1984 | 17.764492825118 | | 1983 | 18.896691452701 | | 1982 | 20.234151138125 | | 1981 | 24.153560026159 | | 1980 | 21.958164955247 | | 1979 | 18.664160818894 | | 1978 | 17.606082142161 | | 1977 | 16.272946140833 | | 1976 | 14.504066458327 | | 1975 | 13.018193752001 | | 1974 | 11.668040565419 | | 1973 | 12.446350275781 | | 1972 | 10.588866847607 | | 1971 | 10.489220762927 | | 1970 | -6.2804690890629 | | 1969 | -6.994056325174 | | 1968 | -7.983780830992 | | 1967 | -7.2812937553408 | | 1966 | -9.2828997482514 | | 1965 | -9.4289775520768 | | 1964 | -19.719407474459 | | 1963 | -17.080467335975 | | 1962 | -14.712290135191 | | 1961 | -17.699936481492 | | 1960 | -19.627350693911 |
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