| Year | Value | | 2021 | 1.4885441653675 | | 2020 | 1.4757381569671 | | 2019 | 1.5189559580597 | | 2018 | 1.5519956476523 | | 2017 | 1.5821962567924 | | 2016 | 1.6247370872832 | | 2015 | 1.6335185024307 | | 2014 | 1.6342730311376 | | 2013 | 1.6255082895198 | | 2012 | 1.6535642050433 | | 2011 | 1.6468784048453 | | 2010 | 1.6635804239437 | | 2009 | 1.6745798971224 | | 2008 | 1.7009511863619 | | 2007 | 1.6878790831467 | | 2006 | 1.6595821319582 | | 2005 | 1.6135408514463 | | 2004 | 1.6138982308499 | | 2003 | 1.599353671177 | | 2002 | 1.5807125220476 | | 2001 | 1.5894597314741 | | 2000 | 1.617485559037 | | 1999 | 1.5869201964246 | | 1998 | 1.5917313364108 | | 1997 | 1.5948781062415 | | 1996 | 1.6042558976233 | | 1995 | 1.6101997841823 | | 1994 | 1.6424144520209 | | 1993 | 1.6638532789476 | | 1992 | 1.703651613947 | | 1991 | 1.7253533921296 | | 1990 | 1.7515905789386 | | 1989 | 1.7336570962636 | | 1988 | 1.739198247249 | | 1987 | 1.719424412638 | | 1986 | 1.7238515676596 | | 1985 | 1.7390100676124 | | 1984 | 1.7405655350788 | | 1983 | 1.7656697654013 | | 1982 | 1.8000824179561 | | 1981 | 1.8075833147516 | | 1980 | 1.851904546661 | | 1979 | 1.8629131621197 | | 1978 | 1.8562373336349 | | 1977 | 1.9009835611482 | | 1976 | 1.9340918099501 | | 1975 | 1.998843302439 | | 1974 | 2.0910291330682 | | 1973 | 2.1454703110338 | | 1972 | 2.2331336659944 | | 1971 | 2.3739468043472 | | 1970 | 2.4378264272491 | | 1969 | 2.4696678740143 | | 1968 | 2.501977277198 | | 1967 | 2.5795594395003 | | 1966 | 2.56389787487 | | 1965 | 2.719205191852 | | 1964 | 2.8131620396388 | | 1963 | 2.8345482038355 | | 1962 | 2.8600875585754 | | 1961 | 2.8964234736125 | | 1960 | 2.8979897995482 |
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