| Year | Value | | 2021 | 2.3845571017627 | | 2020 | 2.4189262645071 | | 2019 | 2.4836908491203 | | 2018 | 2.5229008331214 | | 2017 | 2.5916066541178 | | 2016 | 2.6125094119325 | | 2015 | 2.6086351860897 | | 2014 | 2.6432251863781 | | 2013 | 2.6554684448124 | | 2012 | 2.6931018397625 | | 2011 | 2.6777731179658 | | 2010 | 2.7018445428941 | | 2009 | 2.7305124143459 | | 2008 | 2.7432403444933 | | 2007 | 2.7462250916555 | | 2006 | 2.7532926446845 | | 2005 | 2.7717100129931 | | 2004 | 2.7948199681313 | | 2003 | 2.8090553294196 | | 2002 | 2.8414010261865 | | 2001 | 2.8735878530293 | | 2000 | 2.9149650720149 | | 1999 | 2.9132355818207 | | 1998 | 2.9493210887404 | | 1997 | 2.9943728763592 | | 1996 | 3.0481085633172 | | 1995 | 3.1085184598241 | | 1994 | 3.1715901040038 | | 1993 | 3.2375188697207 | | 1992 | 3.3179527458672 | | 1991 | 3.4238460278404 | | 1990 | 3.6453702593419 | | 1989 | 3.7082503081804 | | 1988 | 3.773785021375 | | 1987 | 3.8985166696277 | | 1986 | 3.9408855073694 | | 1985 | 3.9594841506091 | | 1984 | 4.0120417519774 | | 1983 | 4.0444267325144 | | 1982 | 4.2020489378603 | | 1981 | 4.1864678039727 | | 1980 | 4.21550721437 | | 1979 | 4.252840453093 | | 1978 | 4.2848867912264 | | 1977 | 4.3675533256215 | | 1976 | 4.5247461056968 | | 1975 | 4.6505761414502 | | 1974 | 4.8643328735879 | | 1973 | 5.0604107848994 | | 1972 | 5.2077476650757 | | 1971 | 5.3585248034335 | | 1970 | 5.5424499392524 | | 1969 | 5.5951164026485 | | 1968 | 5.718767082739 | | 1967 | 5.5557084980747 | | 1966 | 5.7067122779115 | | 1965 | 5.8157135598238 | | 1964 | 5.8542906321497 | | 1963 | 6.0954468937535 | | 1962 | 5.710578015379 | | 1961 | 5.0891961725719 | | 1960 | 5.2511619740675 |
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