| Year | Value | | 2021 | 1.7963046417305 | | 2020 | 1.8413699864804 | | 2019 | 1.9040018026138 | | 2018 | 1.8830585849482 | | 2017 | 1.605227579991 | | 2016 | 1.4736367733213 | | 2015 | 1.3645786390266 | | 2014 | 1.3483551149166 | | 2013 | 1.2825597115818 | | 2012 | 1.3907165389815 | | 2011 | 1.4294727354664 | | 2010 | 1.589004055881 | | 2009 | 1.599819738621 | | 2008 | 1.6493916178459 | | 2007 | 1.8008111762055 | | 2006 | 1.7166291122127 | | 2005 | 1.8260477692654 | | 2004 | 2.017124831005 | | 2003 | 2.0405588102749 | | 2002 | 1.99278954484 | | 2001 | 2.1937809824245 | | 2000 | 2.5398828301036 | | 1999 | 2.2857142857143 | | 1998 | 2.2992338891392 | | 1997 | 2.2884182063993 | | 1996 | 2.1928796755295 | | 1995 | 2.1622352410996 | | 1994 | 2.5525011266336 | | 1993 | 2.6345200540784 | | 1992 | 2.7670121676431 | | 1991 | 2.7381703470031 | | 1990 | 2.9788192879675 | | 1989 | 3.1293375394322 | | 1988 | 3.2798557908968 | | 1987 | 3.4465975664714 | | 1986 | 3.4294727354664 | | 1985 | 3.4159531320415 | | 1984 | 3.4015322217215 | | 1983 | 3.3880126182965 | | 1982 | 3.3735917079766 | | 1981 | 3.3600721045516 | | 1980 | 3.3456511942316 | | 1979 | 3.3321315908067 | | 1978 | 3.3177106804867 | | 1977 | 3.3041910770617 | | 1976 | 3.2897701667418 | | 1975 | 3.2762505633168 | | 1974 | 3.2618296529969 | | 1973 | 3.2483100495719 | | 1972 | 3.2338891392519 | | 1971 | 3.220369535827 | | 1970 | 3.2194682289319 | | 1969 | 3.2194682289319 | | 1968 | 3.217665615142 | | 1967 | 3.217665615142 | | 1966 | 3.217665615142 | | 1965 | 3.208652546192 | | 1964 | 3.199639477242 | | 1963 | 3.199639477242 | | 1962 | 3.183415953132 | | 1961 | 3.183415953132 |
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