| Year | Value | | 2021 | 2.6558931068165 | | 2020 | 2.6779301160845 | | 2019 | 2.7083867227004 | | 2018 | 2.7465445446242 | | 2017 | 2.7883845930971 | | 2016 | 2.8374039603423 | | 2015 | 2.8767145762261 | | 2014 | 2.9107619948487 | | 2013 | 2.9424827211349 | | 2012 | 2.9772635670368 | | 2011 | 3.0002615633801 | | 2010 | 3.0144756663488 | | 2009 | 3.0597412346788 | | 2008 | 3.0908632103297 | | 2007 | 3.1095900406812 | | 2006 | 3.1381461828896 | | 2005 | 3.1605840110932 | | 2004 | 3.1941791458431 | | 2003 | 3.2212865395301 | | 2002 | 3.2579335653198 | | 2001 | 3.291450873992 | | 2000 | 3.3453998266475 | | 1999 | 3.4072374683146 | | 1998 | 3.4633975086381 | | 1997 | 3.5222074930607 | | 1996 | 3.5887906485105 | | 1995 | 3.6743187991362 | | 1994 | 3.7601866049181 | | 1993 | 3.8345536102665 | | 1992 | 3.9189144494935 | | 1991 | 3.9601601883685 | | 1990 | 4.0195374069821 | | 1989 | 4.0880260757228 | | 1988 | 4.1397051688392 | | 1987 | 4.1782697627615 | | 1986 | 4.2240758620107 | | 1985 | 4.2642487141387 | | 1984 | 4.3164243566305 | | 1983 | 4.374319789807 | | 1982 | 4.408386456978 | | 1981 | 4.4437461386839 | | 1980 | 4.4782240939142 | | 1979 | 4.5064797675487 | | 1978 | 4.5341932304466 | | 1977 | 4.572471514694 | | 1976 | 4.6175692912127 | | 1975 | 4.6542641336426 | | 1974 | 4.7015991957415 | | 1973 | 4.7639306658642 | | 1972 | 4.863953740833 | | 1971 | 4.9228504646126 | | 1970 | 4.9640237971423 | | 1969 | 5.0181196974524 | | 1968 | 5.0693221946417 | | 1967 | 5.1118182949836 | | 1966 | 5.1700557299843 | | 1965 | 5.2187875154177 | | 1964 | 5.2667987139062 | | 1963 | 5.2850335838899 | | 1962 | 5.3019426983238 | | 1961 | 5.3276170022891 | | 1960 | 5.3579089153035 |
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