| Year | Value | | 2021 | 1.5892184240505 | | 2020 | 1.5885826276679 | | 2019 | 1.6292831046835 | | 2018 | 1.6681551827514 | | 2017 | 1.7016568753536 | | 2016 | 1.7362184170431 | | 2015 | 1.7461119778816 | | 2014 | 1.7552321476045 | | 2013 | 1.7510634158211 | | 2012 | 1.7740425497882 | | 2011 | 1.7742980356122 | | 2010 | 1.7947511942259 | | 2009 | 1.8057987415462 | | 2008 | 1.8331838781607 | | 2007 | 1.8304268484389 | | 2006 | 1.8144507474355 | | 2005 | 1.7837279163844 | | 2004 | 1.7890120865649 | | 2003 | 1.7841730730788 | | 2002 | 1.7822364190007 | | 2001 | 1.8085983201565 | | 2000 | 1.8335654965971 | | 1999 | 1.8169220960152 | | 1998 | 1.8268496846247 | | 1997 | 1.835392312666 | | 1996 | 1.8531897185449 | | 1995 | 1.8617441912141 | | 1994 | 1.9013952261909 | | 1993 | 1.9243781469435 | | 1992 | 1.9608654875135 | | 1991 | 1.9882403157289 | | 1990 | 2.0116118024643 | | 1989 | 2.0028178522672 | | 1988 | 2.012486538084 | | 1987 | 2.0102813471545 | | 1986 | 2.0290257544229 | | 1985 | 2.0567139285233 | | 1984 | 2.0668377390995 | | 1983 | 2.1004498999493 | | 1982 | 2.1466193603236 | | 1981 | 2.1680852330428 | | 1980 | 2.2204199402732 | | 1979 | 2.243685554053 | | 1978 | 2.2488534886506 | | 1977 | 2.2956358523303 | | 1976 | 2.334258303061 | | 1975 | 2.399154077966 | | 1974 | 2.4920149620134 | | 1973 | 2.5501627233508 | | 1972 | 2.6348338798463 | | 1971 | 2.7687267609116 | | 1970 | 2.8326706830239 | | 1969 | 2.867741250818 | | 1968 | 2.9038612559139 | | 1967 | 2.9831996386883 | | 1966 | 2.9718395431378 | | 1965 | 3.1282505397546 | | 1964 | 3.2201144436429 | | 1963 | 3.2364798018261 | | 1962 | 3.2538899677889 | | 1961 | 3.2879672221879 | | 1960 | 3.2855894342703 |
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