| Year | Value | | 2021 | 4.384042086804 | | 2020 | 4.296361245068 | | 2019 | 4.0333187198597 | | 2018 | 3.9456378781236 | | 2017 | 3.9456378781236 | | 2016 | 3.8579570363876 | | 2015 | 3.8579570363876 | | 2014 | 3.4195528277071 | | 2013 | 3.4195528277071 | | 2012 | 3.4195528277071 | | 2011 | 3.288031565103 | | 2010 | 3.288031565103 | | 2009 | 3.2003507233669 | | 2008 | 3.0688294607628 | | 2007 | 3.0688294607628 | | 2006 | 3.0688294607628 | | 2005 | 3.0688294607628 | | 2004 | 3.0688294607628 | | 2003 | 3.0688294607628 | | 2002 | 3.0688294607628 | | 2001 | 2.8496273564226 | | 2000 | 2.8057869355546 | | 1999 | 2.8057869355546 | | 1998 | 2.8057869355546 | | 1997 | 2.8057869355546 | | 1996 | 2.7619465146865 | | 1995 | 2.7181060938185 | | 1994 | 2.6304252520824 | | 1993 | 2.6304252520824 | | 1992 | 2.4550635686103 | | 1991 | 2.3673827268742 | | 1990 | 2.2797018851381 | | 1989 | 2.2797018851381 | | 1988 | 2.192021043402 | | 1987 | 2.148180622534 | | 1986 | 1.9289785181938 | | 1985 | 1.8851380973257 | | 1984 | 2.0166593599299 | | 1983 | 2.0166593599299 | | 1982 | 2.0166593599299 | | 1981 | 2.0166593599299 | | 1980 | 1.9728189390618 | | 1979 | 1.8851380973257 | | 1978 | 1.8851380973257 | | 1977 | 1.8851380973257 | | 1976 | 1.8412976764577 | | 1975 | 1.8412976764577 | | 1974 | 1.7974572555896 | | 1973 | 1.7974572555896 | | 1972 | 1.7536168347216 | | 1971 | 1.7536168347216 | | 1970 | 1.7097764138536 | | 1969 | 1.7097764138536 | | 1968 | 1.6659359929855 | | 1967 | 1.6659359929855 | | 1966 | 1.6659359929855 | | 1965 | 1.6220955721175 | | 1964 | 1.6220955721175 | | 1963 | 1.6220955721175 | | 1962 | 1.5782551512495 | | 1961 | 1.5782551512495 |
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