| Year | Value | | 2021 | 1.6400383996215 | | 2020 | 1.6178563043028 | | 2019 | 1.681889494816 | | 2018 | 1.7072903808243 | | 2017 | 1.7438884382039 | | 2016 | 1.7974698298764 | | 2015 | 1.8191793320272 | | 2014 | 1.8371645255144 | | 2013 | 1.8326418426687 | | 2012 | 1.8552979421682 | | 2011 | 1.867883648582 | | 2010 | 1.9027670835228 | | 2009 | 1.9707423470524 | | 2008 | 2.0348495402957 | | 2007 | 2.0751351540367 | | 2006 | 2.0603660210295 | | 2005 | 2.0095066229951 | | 2004 | 2.0025875250761 | | 2003 | 1.9979841164013 | | 2002 | 1.9706353889855 | | 2001 | 1.981727462238 | | 2000 | 2.0017801371462 | | 1999 | 1.961898321479 | | 1998 | 1.9550532551042 | | 1997 | 1.930704400096 | | 1996 | 1.9411384150453 | | 1995 | 1.9468780942476 | | 1994 | 1.9699200434903 | | 1993 | 1.9860263835998 | | 1992 | 2.0117861755666 | | 1991 | 2.0275357555237 | | 1990 | 2.0553092210204 | | 1989 | 1.9891642477595 | | 1988 | 1.9084274605668 | | 1987 | 1.8527443371437 | | 1986 | 1.821247376818 | | 1985 | 1.8265788443955 | | 1984 | 1.7907726653316 | | 1983 | 1.7870365669556 | | 1982 | 1.8136036630027 | | 1981 | 1.8006723212617 | | 1980 | 1.8294584823162 | | 1979 | 1.8025598855803 | | 1978 | 1.7607492177575 | | 1977 | 1.7891678500474 | | 1976 | 1.7438066573789 | | 1975 | 1.7789832768283 | | 1974 | 1.8352341841723 | | 1973 | 1.8801378391971 | | 1972 | 2.0071147377914 | | 1971 | 2.2538079282998 | | 1970 | 2.4587125923335 | | 1969 | 2.4443927297951 | | 1968 | 2.4566341837554 | | 1967 | 2.5551959275006 | | 1966 | 2.7236550910168 | | 1965 | 2.9319052738638 | | 1964 | 3.2145968620091 | | 1963 | 3.345446639568 | | 1962 | 3.4811214656229 | | 1961 | 3.6321249752086 | | 1960 | 3.6682546485014 |
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