| Year | Value | | 2022 | 2.3309103843889 | | 2021 | 2.3673389620999 | | 2020 | 2.3912190217585 | | 2019 | 2.4116299155391 | | 2018 | 2.4389940347236 | | 2017 | 2.4680788844675 | | 2016 | 2.4861562404835 | | 2015 | 2.5045457411774 | | 2014 | 2.5309987350123 | | 2013 | 2.5597598078264 | | 2012 | 2.6028380501261 | | 2011 | 2.654960815492 | | 2010 | 2.6713169382967 | | 2009 | 2.791410472239 | | 2008 | 2.851778081342 | | 2007 | 2.9074391753854 | | 2006 | 2.8078020633149 | | 2005 | 2.5962511961538 | | 2004 | 2.6255672619467 | | 2003 | 2.6063141565114 | | 2002 | 2.6109178488683 | | 2001 | 2.7064542100931 | | 2000 | 2.8372567506566 | | 1999 | 2.9105959694942 | | 1998 | 2.9129670025605 | | 1997 | 3.2456925957889 | | 1996 | 3.8126896823235 | | 1995 | 4.4668205559347 | | 1994 | 0.34806039394431 | | 1993 | 0.6799860670771 | | 1992 | 2.8657539593188 | | 1991 | 2.90550941174 | | 1990 | 2.9398928934769 | | 1989 | 2.9727375911534 | | 1988 | 3.0146147809694 | | 1987 | 3.0229252310095 | | 1986 | 3.0363563997517 | | 1985 | 3.1748118222068 | | 1984 | 3.321928649178 | | 1983 | 3.3594173559228 | | 1982 | 3.2773009346653 | | 1981 | 3.0451825784449 | | 1980 | 2.7458483479216 | | 1979 | 2.4875518163616 | | 1978 | 2.3582495353379 | | 1977 | 2.3443965532191 | | 1976 | 2.3609471171641 | | 1975 | 2.3943654073417 | | 1974 | 2.4551121540643 | | 1973 | 2.5503260499065 | | 1972 | 2.7619926121384 | | 1971 | 3.1283334903132 | | 1970 | 3.5621799144698 | | 1969 | 3.9301075318817 | | 1968 | 4.0847745735315 | | 1967 | 3.8918116954072 | | 1966 | 3.3524309847386 | | 1965 | 2.6577329507313 | | 1964 | 2.0491696975974 | | 1963 | 1.7340739751368 | | 1962 | 1.6572189115089 | | 1961 | 1.6468674732083 |
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