| Year | Value | | 2022 | 2.2741694922287 | | 2021 | 2.1940321108583 | | 2020 | 2.3378113437568 | | 2019 | 2.1813559840656 | | 2018 | 2.1345916857463 | | 2017 | 2.1641769880828 | | 2016 | 2.2130094372543 | | 2015 | 2.2582044476951 | | 2014 | 2.2508194723231 | | 2013 | 2.3009388565036 | | 2012 | 2.3679461337649 | | 2011 | 2.4056199267519 | | 2010 | 2.516327772915 | | 2009 | 2.6133763987256 | | 2008 | 2.3818931873498 | | 2007 | 2.3240339278076 | | 2006 | 2.362450732173 | | 2005 | 2.3921403770412 | | 2004 | 2.4007472060773 | | 2003 | 2.401898156517 | | 2002 | 2.341180084432 | | 2001 | 2.2384648745778 | | 2000 | 2.202398287704 | | 1999 | 2.2147238622626 | | 1998 | 2.2576471022339 | | 1997 | 2.3091922591732 | | 1996 | 2.3058873573246 | | 1995 | 2.3790335913702 | | 1994 | 2.5781230728531 | | 1993 | 2.7490883224312 | | 1992 | 2.9245860965462 | | 1991 | 3.0023917575921 | | 1990 | 3.3012975250631 | | 1989 | 3.4280139645726 | | 1988 | 3.5504465754457 | | 1987 | 3.8169014675312 | | 1986 | 3.9585275944713 | | 1985 | 4.2778112554944 | | 1984 | 4.2185601277268 | | 1983 | 4.2229227780653 | | 1982 | 4.4114123934913 | | 1981 | 3.8601140738844 | | 1980 | 3.5739184103623 | | 1979 | 3.5425386151919 | | 1978 | 3.6065055941409 | | 1977 | 3.8007888001148 | | 1976 | 3.7066995556536 | | 1975 | 3.8646184146818 | | 1974 | 3.8621966528287 | | 1973 | 3.8265425311075 | | 1972 | 4.2810733959817 | | 1971 | 4.5541103646009 | | 1970 | 5.0441877368895 | | 1969 | 5.7019720062393 | | 1968 | 6.1360915394554 | | 1967 | 6.2289624274061 | | 1966 | 5.7989128559058 | | 1965 | 5.416894118936 | | 1964 | 5.6695612729336 | | 1963 | 6.1438473888451 | | 1962 | 6.496369736652 | | 1961 | 6.3780711919649 | | 1960 | 6.4228800043017 |
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