| Year | Value | | 2022 | 1.2391466836125 | | 2021 | 1.5150448644861 | | 2020 | 1.8852204974286 | | 2019 | 1.9263605288841 | | 2018 | 1.9203443717683 | | 2017 | 1.8843564380887 | | 2016 | 1.8437998929318 | | 2015 | 1.8160160415618 | | 2014 | 1.8025758189554 | | 2013 | 1.8039158028739 | | 2012 | 2.0391269983749 | | 2011 | 2.4296152118737 | | 2010 | 2.5950318244663 | | 2009 | 2.6087730292484 | | 2008 | 2.6252790773745 | | 2007 | 2.6480298074104 | | 2006 | 2.6744129396537 | | 2005 | 2.7155176123439 | | 2004 | 2.770996280051 | | 2003 | 2.8215044674889 | | 2002 | 2.3725582288621 | | 2001 | 1.6592460274269 | | 2000 | 1.4877568229253 | | 1999 | 1.5971079527374 | | 1998 | 1.704124198818 | | 1997 | 1.8145172389165 | | 1996 | 1.9118166925098 | | 1995 | 2.0020530679711 | | 1994 | 2.0967875007622 | | 1993 | 2.215520015557 | | 1992 | 2.716700364957 | | 1991 | 3.4139464288646 | | 1990 | 3.7502049394081 | | 1989 | 3.8861729050714 | | 1988 | 3.9830123468413 | | 1987 | 4.0303246746749 | | 1986 | 4.0166096463017 | | 1985 | 3.937838981717 | | 1984 | 3.8092896174543 | | 1983 | 3.6570061958121 | | 1982 | 3.1649676544035 | | 1981 | 1.3614850800244 | | 1980 | 0.11121591085595 | | 1979 | 0.18020844778214 | | 1978 | 0.14498848115026 | | 1977 | 0.10384006319771 | | 1976 | 0.10454233295415 | | 1975 | 0.14609502010694 | | 1974 | 0.22540090897357 | | 1973 | 0.33473879560471 | | 1972 | 0.4665997099397 | | 1971 | 1.0251218281186 | | 1970 | 2.8166717501105 | | 1969 | 3.1637070087979 | | 1968 | 3.3962397877598 | | 1967 | 3.5805381363247 | | 1966 | 3.730239520968 | | 1965 | 3.8469841160777 | | 1964 | 3.9306524760307 | | 1963 | 3.9930816573843 | | 1962 | 4.0404606449882 | | 1961 | 0.93228972123367 |
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