| Year | Value | | 2021 | 1.8640224462816 | | 2020 | 1.8947398658785 | | 2019 | 1.9342593156613 | | 2018 | 1.9830162426162 | | 2017 | 2.0105776204724 | | 2016 | 2.0299594505464 | | 2015 | 2.0811398088231 | | 2014 | 2.105480146013 | | 2013 | 2.1235148026403 | | 2012 | 2.1499434538575 | | 2011 | 2.1819680400988 | | 2010 | 2.2027173144776 | | 2009 | 2.226587004577 | | 2008 | 2.2509163700681 | | 2007 | 2.2876837011258 | | 2006 | 2.3295456135603 | | 2005 | 2.372114299158 | | 2004 | 2.409272202879 | | 2003 | 2.4458998549721 | | 2002 | 2.5086134812283 | | 2001 | 2.5727520414872 | | 2000 | 2.6380186125151 | | 1999 | 2.7031993501521 | | 1998 | 2.7639964350475 | | 1997 | 2.8269628038253 | | 1996 | 2.8958840252365 | | 1995 | 2.9690907252846 | | 1994 | 3.0335989286211 | | 1993 | 3.0996090857711 | | 1992 | 3.1593703951964 | | 1991 | 3.2366855174698 | | 1990 | 3.3128184433493 | | 1989 | 3.3936544881844 | | 1988 | 3.4734901513718 | | 1987 | 3.5690235475409 | | 1986 | 3.6638379043112 | | 1985 | 3.759172184615 | | 1984 | 3.8543832537073 | | 1983 | 3.9519628165306 | | 1982 | 4.0555471233673 | | 1981 | 4.1584626718875 | | 1980 | 4.2594146980873 | | 1979 | 4.3669156673489 | | 1978 | 4.4754030177223 | | 1977 | 4.5647537801429 | | 1976 | 4.6573015347879 | | 1975 | 4.7493045596283 | | 1974 | 4.8494521484814 | | 1973 | 4.9646081778382 | | 1972 | 5.0614362819651 | | 1971 | 5.1607185611591 | | 1970 | 5.2611379005618 | | 1969 | 5.3568972507645 | | 1968 | 5.4508954913654 | | 1967 | 5.5599851959873 | | 1966 | 5.6613531608509 | | 1965 | 5.7493293027191 | | 1964 | 5.8286540494382 | | 1963 | 5.884338700212 | | 1962 | 5.917019302881 | | 1961 | 5.9355567715236 | | 1960 | 5.9454407327123 |
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