| Year | Value | | 2021 | 2.859601735606 | | 2020 | 2.8876899943085 | | 2019 | 2.9234709353677 | | 2018 | 2.9694557617147 | | 2017 | 3.0179148755451 | | 2016 | 3.0724549036073 | | 2015 | 3.1153518516462 | | 2014 | 3.1606978736957 | | 2013 | 3.199352870702 | | 2012 | 3.2366271005773 | | 2011 | 3.2644590339631 | | 2010 | 3.2813193569086 | | 2009 | 3.3263113777413 | | 2008 | 3.3651702511642 | | 2007 | 3.3933482881102 | | 2006 | 3.4152400687207 | | 2005 | 3.4458826620777 | | 2004 | 3.4827702177476 | | 2003 | 3.5151507798933 | | 2002 | 3.5566895723054 | | 2001 | 3.5910994918544 | | 2000 | 3.657218381358 | | 1999 | 3.7061279930046 | | 1998 | 3.7570550356426 | | 1997 | 3.82134420165 | | 1996 | 3.9069101584753 | | 1995 | 3.9927957619855 | | 1994 | 4.0980699652181 | | 1993 | 4.1865800765121 | | 1992 | 4.2939691235059 | | 1991 | 4.3564844542602 | | 1990 | 4.4421702507874 | | 1989 | 4.5095733156768 | | 1988 | 4.5617213018203 | | 1987 | 4.5845056584623 | | 1986 | 4.6191773075753 | | 1985 | 4.6448444358339 | | 1984 | 4.6937555905535 | | 1983 | 4.737801111478 | | 1982 | 4.7508364883102 | | 1981 | 4.7732076928881 | | 1980 | 4.791092096335 | | 1979 | 4.7944674557423 | | 1978 | 4.7991937169284 | | 1977 | 4.8120633979413 | | 1976 | 4.835432926306 | | 1975 | 4.8184194778284 | | 1974 | 4.8101639332994 | | 1973 | 4.8148124067179 | | 1972 | 4.8779649636524 | | 1971 | 4.8964497707489 | | 1970 | 4.9000798812004 | | 1969 | 4.9237049512979 | | 1968 | 4.9341336326436 | | 1967 | 4.9442402799485 | | 1966 | 4.9771909822051 | | 1965 | 5.0126007865626 | | 1964 | 5.0443702424073 | | 1963 | 5.0583382491986 | | 1962 | 5.074290583547 | | 1961 | 5.07441799411 | | 1960 | 5.0697702609664 |
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