| Year | Value | | 2021 | 4.0646020161154 | | 2020 | 3.9838689967005 | | 2019 | 3.9746386514856 | | 2018 | 4.0267706012205 | | 2017 | 3.9979095218161 | | 2016 | 3.9075561425997 | | 2015 | 4.0470562201424 | | 2014 | 4.1602514871848 | | 2013 | 4.0639912526197 | | 2012 | 4.118219803965 | | 2011 | 4.1257045416086 | | 2010 | 3.331261471174 | | 2009 | 3.5330044387748 | | 2008 | 3.1232964086276 | | 2007 | 3.0101662262604 | | 2006 | 3.1537430196686 | | 2005 | 3.4367311872746 | | 2004 | 3.3509495854106 | | 2003 | 3.0251357015477 | | 2002 | 3.0895695299585 | | 2001 | 3.1500982778595 | | 2000 | 3.0554651601734 | | 1999 | 2.9931140413678 | | 1998 | 2.7791156294339 | | 1997 | 2.721060099189 | | 1996 | 2.4983403407834 | | 1995 | 2.1916613514182 | | 1994 | 2.3226117178449 | | 1993 | 2.2273277534072 | | 1992 | 2.1121278783698 | | 1991 | 2.2399541803887 | | 1990 | 2.2071515041068 | | 1989 | 2.2814782031423 | | 1988 | 2.3607513374901 | | 1987 | 2.5513192663655 | | 1986 | 2.6915116566653 | | 1985 | 2.7287400908582 | | 1984 | 2.8297515067102 | | 1983 | 2.5056298244016 | | 1982 | 2.5308826783645 | | 1981 | 2.3724665790193 | | 1980 | 2.3151920648764 | | 1979 | 2.2480246801088 | | 1978 | 2.1653671426526 | | 1977 | 1.9321036668706 | | 1976 | 1.8838108379001 | | 1975 | 1.7667885919581 | | 1974 | 1.9236426590995 | | 1973 | 1.9426473842469 | | 1972 | 2.2189969149864 | | 1971 | 2.1942647384247 | | 1970 | 2.4073494396209 | | 1969 | 2.5560110904287 | | 1968 | 2.787116215196 | | 1967 | 2.5088629056402 | | 1966 | 2.445278705596 | | 1965 | 2.2555270166487 | | 1964 | 2.1212468791898 | | 1963 | 2.0024545253375 | | 1962 | 2.0196807336345 | | 1961 | 1.8716512372597 |
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