| Year | Value | | 2022 | 3.0791077536106 | | 2021 | 3.2375891040639 | | 2020 | 3.5335264473791 | | 2019 | 3.1456566937215 | | 2018 | 3.0332327385044 | | 2017 | 3.2080967243944 | | 2016 | 3.0711310261154 | | 2015 | 3.1099600841067 | | 2014 | 3.1082588481104 | | 2013 | 3.2722628565187 | | 2012 | 3.1560058634043 | | 2011 | 3.0771069890655 | | 2010 | 3.6369150461346 | | 2009 | 3.8869638370635 | | 2008 | 3.7372469856511 | | 2007 | 3.2863017094945 | | 2006 | 3.2958249913523 | | 2005 | 3.3746796939265 | | 2004 | 3.4650151740631 | | 2003 | 3.463988690815 | | 2002 | 3.417127623582 | | 2001 | 3.3238728188053 | | 2000 | 3.0316835386585 | | 1999 | 3.762742057863 | | 1998 | 3.4409857420471 | | 1997 | 2.8017878857594 | | 1996 | 4.4453932539643 | | 1995 | 2.831626580577 | | 1994 | 2.5084070134705 | | 1993 | 2.525267018291 | | 1992 | 2.1066779425534 | | 1991 | 1.8534817831972 | | 1990 | 1.8601592995481 | | 1989 | 2.1881838074398 | | 1988 | 2.1992447554832 | | 1987 | 1.9831354164188 | | 1986 | 1.9446195911585 | | 1985 | 2.0540166093421 | | 1984 | 2.3258949639318 | | 1983 | 2.2199375274218 | | 1982 | 1.7138509590358 | | 1981 | 1.7551203619179 | | 1980 | 1.8301216492626 | | 1979 | 1.6739287697044 | | 1978 | 1.5503194097791 | | 1977 | 1.4384872142229 | | 1976 | 1.4936028707235 | | 1975 | 1.7402680753376 | | 1974 | 1.5757801352441 | | 1973 | 1.690245106103 | | 1972 | 1.9671958230051 | | 1971 | 3.3741740971198 | | 1970 | 2.2595465843188 | | 1969 | 2.0549241570755 | | 1968 | 2.3853724810985 | | 1967 | 3.1173645631477 | | 1966 | 3.165244797044 | | 1965 | 3.1909469217231 | | 1964 | 3.1808035714286 | | 1963 | 3.5151403758673 | | 1962 | 3.0994152046784 | | 1961 | 2.1432563032116 | | 1960 | 1.88432347553 |
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