| Year | Value | | 2022 | 8.3042406016444 | | 2021 | 4.2663575447981 | | 2020 | 2.0024120575929 | | 2019 | 2.1740841334292 | | 2018 | 2.7823570914923 | | 2017 | 3.2795569467855 | | 2016 | 2.7246122329529 | | 2015 | 3.1290027447393 | | 2014 | 3.4381665995861 | | 2013 | 2.76789666082 | | 2012 | 4.1115098107029 | | 2011 | 5.1289237668162 | | 2010 | 3.8595090983317 | | 2009 | 2.5919467343057 | | 2008 | 10.066241824585 | | 2007 | 6.1435665524805 | | 2006 | 4.2823960480256 | | 2005 | 5.051019626897 | | 2004 | 5.904020055465 | | 2003 | 5.6034767543447 | | 2002 | 5.2233676975945 | | 2001 | 6.8019093078759 | | 2000 | 7.0699135225252 | | 1999 | 5.0360290425189 | | 1998 | 6.9306412286576 | | 1997 | 7.7208395783799 | | 1996 | 11.297429471858 | | 1995 | 12.980686910268 | | 1994 | 12.194894938391 | | 1993 | 11.284168445499 | | 1992 | 13.626533683001 | | 1991 | 25.096407309349 | | 1990 | 22.641370776577 | | 1989 | 14.751503572715 | | 1988 | 13.417397411965 | | 1987 | 14.064711115851 | | 1986 | 16.89666805447 | | 1985 | 20.507583176283 | | 1984 | 11.727298258809 | | 1983 | 10.91288721854 | | 1982 | 7.7294922391132 | | 1981 | 13.657328082689 | | 1980 | 18.135170796071 | | 1979 | 14.842308165533 | | 1978 | 10.507959934319 | | 1977 | 10.688957420775 | | 1976 | 9.7936532753465 | | 1975 | 14.944534640927 | | 1974 | 23.244048532597 | | 1973 | 13.190707015634 | | 1972 | 5.1856976800431 | | 1971 | 4.3139356210801 | | 1970 | 4.3039033520146 | | 1969 | 2.4418611681362 | | 1968 | 3.2535674094159 | | 1967 | 2.5543947308473 | | 1966 | 2.8731942216033 | | 1965 | 2.7459324879856 | | 1964 | 3.6791188489122 | | 1963 | 2.5147602842078 | | 1962 | 2.0766952841157 | | 1961 | 3.1052501467592 | | 1960 | 2.2668709916278 |
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