| Year | Value | | 2022 | 26.613263392649 | | 2021 | 27.348875776558 | | 2020 | 28.171908217426 | | 2019 | 31.700745281597 | | 2018 | 31.686584944473 | | 2017 | 31.942467231779 | | 2016 | 30.076504447869 | | 2015 | 28.043823657924 | | 2014 | 27.485078954706 | | 2013 | 27.220767146219 | | 2012 | 28.931311396432 | | 2011 | 31.433962966498 | | 2010 | 29.627590679385 | | 2009 | 29.274060796544 | | 2008 | 30.662055726251 | | 2007 | 32.249834091558 | | 2006 | 30.445148312901 | | 2005 | 28.240952423302 | | 2004 | 27.899484106247 | | 2003 | 27.973839641845 | | 2002 | 28.103890614555 | | 2001 | 28.225509577269 | | 2000 | 30.782093724527 | | 1999 | 30.455237567541 | | 1998 | 31.743744505646 | | 1997 | 32.254597509117 | | 1996 | 34.603520180903 | | 1995 | 35.802811705384 | | 1994 | 35.802707100538 | | 1993 | 35.940378563963 | | 1992 | 34.019905421832 | | 1991 | 34.528016169832 | | 1990 | 32.826818710886 | | 1989 | 34.049263961338 | | 1988 | 32.009949938309 | | 1987 | 27.308516457679 | | 1986 | 24.397629444387 | | 1985 | 23.009720208625 | | 1984 | 22.439519881284 | | 1983 | 21.991578145991 | | 1982 | 22.303579650492 | | 1981 | 20.852367918043 | | 1980 | 22.094621176911 | | 1979 | 22.003213630475 | | 1978 | 23.73005896787 | | 1977 | 21.887982375049 | | 1976 | 20.456565309281 | | 1975 | 20.561156730218 |
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