| Year | Value | | 2022 | 4.779438286484 | | 2021 | 5.5854466677517 | | 2020 | 6.6924683973574 | | 2019 | 5.9996902593765 | | 2018 | 5.4835887005616 | | 2017 | 5.9243833268853 | | 2016 | 6.3849289830186 | | 2015 | 6.2702428562351 | | 2014 | 5.5484833358485 | | 2013 | 4.8445089170406 | | 2012 | 4.461078620077 | | 2011 | 4.3257063586107 | | 2010 | 3.5180126088262 | | 2009 | 3.8485136982934 | | 2008 | 3.0247471409039 | | 2007 | 2.9233210269767 | | 2006 | 2.6438075477202 | | 2005 | 2.8341783501146 | | 2004 | 3.2838845233139 | | 2003 | 3.2512108934718 | | 2002 | 3.700829358999 | | 2001 | 3.8206954633307 | | 2000 | 3.4333856350536 | | 1999 | 3.7550799827064 | | 1998 | 3.9656737879307 | | 1997 | 3.6374034681332 | | 1996 | 3.0940883958553 | | 1995 | 2.9350271073671 | | 1994 | 3.1464300121016 | | 1993 | 2.5056315771778 | | 1992 | 2.1401321298967 | | 1991 | 1.2108340592461 | | 1990 | 1.4610705859434 | | 1989 | 1.5401237785634 | | 1988 | 1.7496987492702 | | 1987 | 1.8563762767584 | | 1986 | 1.787213666452 | | 1985 | 1.6437068968474 | | 1984 | 1.7551240070341 | | 1983 | 1.9512132516513 | | 1982 | 2.0630974406414 | | 1981 | 1.8180488747526 | | 1980 | 2.102063295735 | | 1979 | 2.1386958657963 | | 1978 | 2.3752289377289 | | 1977 | 2.2419504590731 | | 1976 | 2.7017212284846 | | 1975 | 2.1310942930458 | | 1974 | 1.9576681485214 | | 1973 | 1.6881619937695 | | 1972 | 1.7958439664601 | | 1971 | 2.0875850340136 | | 1970 | 2.1305391835844 | | 1969 | 2.3868673583711 | | 1968 | 2.6147278548559 | | 1967 | 3.0191004313001 | | 1966 | 3.5125936010892 | | 1965 | 3.4186351706037 | | 1964 | 3.4964539007092 | | 1963 | 2.5114854517611 |
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