| Year | Value | | 2022 | 1.8277683413342 | | 2021 | 1.7397575056098 | | 2020 | 1.898271960686 | | 2019 | 1.8029935144675 | | 2018 | 1.7855401591575 | | 2017 | 1.8568147401118 | | 2016 | 1.9178221656497 | | 2015 | 1.9153295966982 | | 2014 | 1.9005720731319 | | 2013 | 1.8907224947582 | | 2012 | 1.8565876503918 | | 2011 | 1.8116727349854 | | 2010 | 1.9061403637398 | | 2009 | 2.0565387097889 | | 2008 | 1.879925520951 | | 2007 | 1.8717511985038 | | 2006 | 1.9064073156123 | | 2005 | 1.9545365689535 | | 2004 | 1.9836365522969 | | 2003 | 2.0391801111219 | | 2002 | 2.0773997875653 | | 2001 | 2.0345320817317 | | 2000 | 1.9697723729682 | | 1999 | 2.0524927693043 | | 1998 | 1.9489823963656 | | 1997 | 2.0677731507634 | | 1996 | 2.09204743653 | | 1995 | 2.1961136197271 | | 1994 | 2.294552087707 | | 1993 | 2.3180518311922 | | 1992 | 2.4756929447731 | | 1991 | 2.3924735671512 | | 1990 | 2.6021334228411 | | 1989 | 2.8415463386112 | | 1987 | 3.4936786054269 | | 1986 | 3.5607170806103 | | 1985 | 3.3700876176609 | | 1984 | 3.3514980272344 | | 1983 | 3.5428243043531 | | 1982 | 3.551322744862 | | 1981 | 3.4096466140495 | | 1980 | 3.155853313355 | | 1979 | 3.3902392686282 | | 1978 | 4.0801611278051 | | 1977 | 4.3772370181466 | | 1976 | 4.4591217694341 | | 1975 | 4.4076767540194 | | 1974 | 3.7073977055913 | | 1973 | 3.1236776836594 | | 1972 | 3.4296104746093 | | 1971 | 3.5652133828704 | | 1970 | 3.4361293395965 | | 1969 | 3.3279382044223 | | 1968 | 3.2448678506125 | | 1965 | 3.1276584669735 | | 1964 | 2.8967966675943 | | 1963 | 2.8451149202748 | | 1962 | 2.521702946156 |
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