| Year | Value | | 2022 | 4.318 | | 2021 | 5.174 | | 2020 | 5.808 | | 2019 | 2.917 | | 2018 | 2.805 | | 2017 | 3.124 | | 2016 | 3.392 | | 2015 | 3.314 | | 2014 | 3.296 | | 2013 | 3.4 | | 2012 | 3.285 | | 2011 | 3.422 | | 2010 | 4.329 | | 2009 | 5.26 | | 2008 | 3.56 | | 2007 | 4.01 | | 2006 | 4.77 | | 2005 | 5.58 | | 2004 | 6.74 | | 2003 | 7.86 | | 2002 | 7.28 | | 2001 | 5.09 | | 2000 | 4.92 | | 1999 | 6.25 | | 1998 | 4.58 | | 1997 | 2.22 | | 1996 | 2.83 | | 1995 | 3.22 | | 1994 | 1.9 | | 1993 | 1.96 | | 1992 | 1.96 | | 1991 | 1.8 | | 1990 | 1.32 | | 1989 | 1.07 | | 1988 | 1.36 | | 1987 | 1.73 | | 1986 | 2.82 | | 1985 | 3.18 | | 1984 | 1.92 | | 1983 | 2.11 | | 1982 | 1.85 | | 1981 | 1.53 | | 1980 | 2.4 | | 1979 | 2.09 | | 1978 | 1.59 | | 1977 | 4.3 | | 1976 | 5 | | 1975 | 9.17 |
|