| Year | Value | | 2023 | 5.415 | | 2022 | 5.28 | | 2021 | 7.527 | | 2020 | 9.657 | | 2019 | 5.69 | | 2018 | 5.837 | | 2017 | 6.426 | | 2016 | 7.038 | | 2015 | 6.945 | | 2014 | 7.023 | | 2013 | 7.143 | | 2012 | 7.392 | | 2011 | 7.637 | | 2010 | 8.178 | | 2009 | 8.46 | | 2008 | 6.284 | | 2007 | 6.156 | | 2006 | 6.484 | | 2005 | 6.758 | | 2004 | 7.185 | | 2003 | 7.574 | | 2002 | 7.665 | | 2001 | 7.219 | | 2000 | 6.829 | | 1999 | 7.583 | | 1998 | 8.279 | | 1997 | 9.101 | | 1996 | 9.62 | | 1995 | 9.489 | | 1994 | 10.395 | | 1993 | 11.377 | | 1992 | 11.197 | | 1991 | 10.316 | | 1990 | 8.131 | | 1989 | 7.547 | | 1988 | 7.762 | | 1987 | 8.82 | | 1986 | 9.593 | | 1985 | 10.503 | | 1984 | 11.344 | | 1983 | 12.018 | | 1982 | 11.04 | | 1981 | 7.607 | | 1980 | 7.537 | | 1979 | 7.525 | | 1978 | 8.376 | | 1977 | 8.049 | | 1976 | 7.09 | | 1974 | 5.4 | | 1973 | 5.6 | | 1972 | 6.3 | | 1971 | 6.4 | | 1970 | 5.9 | | 1969 | 4.7 |
|