| Year | Value | | 2022 | 3.552 | | 2021 | 5.089 | | 2020 | 7.903 | | 2019 | 3.682 | | 2018 | 3.519 | | 2017 | 3.655 | | 2016 | 3.498 | | 2015 | 3.111 | | 2014 | 2.021 | | 2013 | 2.437 | | 2012 | 2.066 | | 2011 | 2.255 | | 2009 | 2.973 | | 2008 | 2.603 | | 2007 | 5.18 | | 2006 | 5.08 | | 2005 | 5.45 | | 2004 | 4.17 | | 2002 | 5.48 | | 2001 | 5.24 | | 2000 | 4.47 | | 1999 | 3.93 | | 1997 | 2.08 | | 1996 | 5.23 | | 1995 | 3.69 | | 1994 | 3.08 | | 1993 | 5.99 | | 1992 | 2.52 | | 1991 | 6.59 | | 1990 | 6.69 | | 1989 | 10.1 | | 1988 | 18 | | 1987 | 20.32 | | 1986 | 19.76 | | 1985 | 19.28 | | 1984 | 14.66 | | 1983 | 14.19 | | 1982 | 11.6 | | 1981 | 10.16 | | 1980 | 6.1 | | 1979 | 5.6 | | 1978 | 5.5 | | 1977 | 5.3 | | 1976 | 5.3 | | 1975 | 5.2 | | 1974 | 6.1 | | 1973 | 7.1 | | 1972 | 8.1 | | 1971 | 9 |
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