| Year | Value | | 2022 | 2.5591382705715 | | 2021 | 2.6114177663744 | | 2020 | 2.6657717830439 | | 2019 | 2.6828575601837 | | 2018 | 2.6855614271499 | | 2017 | 2.6954663684374 | | 2016 | 2.71781408401 | | 2015 | 2.759113701331 | | 2014 | 2.7888132387524 | | 2013 | 2.806762967711 | | 2012 | 2.8325655537982 | | 2011 | 2.8642770464124 | | 2010 | 2.8915057305237 | | 2009 | 2.9057233935421 | | 2008 | 2.8774060341907 | | 2007 | 2.8344819329295 | | 2006 | 2.8034581645723 | | 2005 | 2.736342194188 | | 2004 | 2.6400402435105 | | 2003 | 2.5425390572923 | | 2002 | 2.4540691481821 | | 2001 | 2.3710066148072 | | 2000 | 2.3009352178142 | | 1999 | 2.2252407740807 | | 1998 | 2.0690464031732 | | 1997 | 1.9416448246573 | | 1996 | 1.9361978781419 | | 1995 | 0.18753634547836 | | 1994 | -1.2304221455031 | | 1993 | 1.3922123012061 | | 1992 | 2.8460931702224 | | 1991 | 3.0141691007144 | | 1990 | 3.8987419806195 | | 1989 | 4.6870998586063 | | 1988 | 5.3827324000662 | | 1987 | 4.7667728056287 | | 1986 | 3.6662347141098 | | 1985 | 3.8239685869771 | | 1984 | 3.8996228261836 | | 1983 | 3.9588590872096 | | 1982 | 3.9760388938242 | | 1981 | 3.9501008651402 | | 1980 | 3.8752269020316 | | 1979 | 3.7228649727311 | | 1978 | 3.4519888288002 | | 1977 | 3.1402937576828 | | 1976 | 2.9344058573378 | | 1975 | 2.8185003272573 | | 1974 | 2.7238413507863 | | 1973 | 2.6417706205907 | | 1972 | 2.5707517550716 | | 1971 | 2.5053348646939 | | 1970 | 2.4575536527001 | | 1969 | 2.4547537768152 | | 1968 | 2.4754827391552 | | 1967 | 2.474589908499 | | 1966 | 2.4558892931876 | | 1965 | 2.4442460283077 | | 1964 | 2.4540332200006 | | 1963 | 2.4562697215754 | | 1962 | 2.4233977612252 | | 1961 | 2.3772095927017 |
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