| Year | Value | | 2022 | 7.392 | | 2021 | 8.722 | | 2020 | 8.291 | | 2019 | 6.833 | | 2018 | 6.365 | | 2017 | 6.718 | | 2016 | 6.99 | | 2015 | 7.432 | | 2014 | 7.954 | | 2013 | 8.052 | | 2012 | 7.976 | | 2011 | 7.804 | | 2010 | 8.61 | | 2009 | 8.351 | | 2008 | 6.235 | | 2007 | 6.161 | | 2006 | 7.066 | | 2005 | 7.486 | | 2004 | 6.693 | | 2003 | 5.555 | | 2002 | 4.965 | | 2001 | 4.73 | | 2000 | 5.466 | | 1999 | 7.607 | | 1998 | 8.941 | | 1997 | 10.359 | | 1996 | 9.547 | | 1995 | 8.897 | | 1994 | 9.58 | | 1993 | 9.33 | | 1992 | 5.722 | | 1991 | 3.243 | | 1990 | 1.832 | | 1989 | 1.46 | | 1987 | 2.15 | | 1986 | 2.23 | | 1985 | 2.83 | | 1984 | 3.07 | | 1983 | 3.48 | | 1982 | 3.14 | | 1976 | 1.61 | | 1975 | 1.62 | | 1974 | 1.98 | | 1973 | 2.46 | | 1972 | 2.7 | | 1971 | 2.55 | | 1970 | 1.51 | | 1969 | 1.89 |
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