| Year | Value | | 2021 | 32.244615880619 | | 2020 | 32.166543846721 | | 2019 | 32.01915991157 | | 2018 | 32.037582903464 | | 2017 | 32.139646278556 | | 2016 | 32.203389830508 | | 2015 | 32.329587324982 | | 2014 | 32.111274871039 | | 2013 | 32.046794399411 | | 2012 | 30.554532056006 | | 2011 | 29.937361827561 | | 2010 | 29.347826086956 | | 2009 | 28.979366249079 | | 2008 | 28.432004419483 | | 2007 | 28.404382653531 | | 2006 | 28.831814415907 | | 2005 | 29.206553755523 | | 2004 | 30.461566752483 | | 2003 | 30.55070331893 | | 2002 | 30.836397058824 | | 2001 | 31.338696556088 | | 2000 | 31.872005785049 | | 1999 | 32.342041037693 | | 1998 | 31.962397179788 | | 1997 | 38.850221458917 | | 1996 | 37.991503208894 | | 1995 | 36.13847961674 | | 1994 | 36.165597035162 | | 1993 | 36.726023682545 | | 1992 | 36.581397450963 | | 1991 | 34.927234927235 | | 1990 | 34.854921811444 | | 1989 | 34.782608695652 | | 1988 | 34.710295579861 | | 1987 | 34.574708487752 | | 1986 | 34.520473650908 | | 1985 | 34.439121395643 | | 1984 | 34.412003977221 | | 1983 | 34.457199674591 | | 1982 | 34.511434511435 | | 1981 | 34.565669348278 | | 1980 | 34.5927867667 | | 1979 | 35.225526529874 | | 1978 | 35.487661574618 | | 1977 | 35.587092108831 | | 1976 | 35.614209527253 | | 1975 | 35.76787489831 | | 1974 | 37.042393564133 | | 1973 | 37.241254632559 | | 1972 | 37.31356774835 | | 1971 | 37.349724306246 | | 1970 | 37.440115700985 | | 1969 | 37.584741932568 | | 1968 | 37.548585374672 | | 1967 | 37.476272258881 | | 1966 | 37.657055048359 | | 1965 | 37.666094187833 | | 1964 | 37.783603000994 | | 1963 | 37.40395914309 | | 1962 | 37.548585374672 | | 1961 | 38.434421043117 |
|