| Year | Value | | 2022 | 3.0693843516053 | | 2021 | 3.1462365877738 | | 2020 | 3.4181973476083 | | 2019 | 3.6331649044943 | | 2018 | 3.6130589575193 | | 2017 | 3.9197847769736 | | 2016 | 3.7709543827142 | | 2015 | 3.3591503328892 | | 2014 | 3.492097731424 | | 2013 | 3.2593885103277 | | 2012 | 1.8128210486869 | | 2011 | 1.5690144701217 | | 2010 | 2.4972120601753 | | 2009 | 2.4611222297499 | | 2008 | 2.8967903301489 | | 2007 | 3.0419915005687 | | 2006 | 2.9898428676581 | | 2005 | 3.4003180462892 | | 2004 | 3.613590935783 | | 2003 | 3.622609540762 | | 2002 | 3.6835542293635 | | 2001 | 3.9267391765521 | | 2000 | 3.9404969816587 | | 1999 | 3.9858107895156 | | 1998 | 4.0857487669839 | | 1997 | 3.4443739062774 | | 1996 | 3.5624993456479 | | 1995 | 3.5443169098732 | | 1994 | 4.9949276665305 | | 1993 | 2.9520101814156 | | 1992 | -4.6291166049018 | | 1991 | -3.8804448891668 | | 1990 | 0.51704226076454 | | 1989 | -1.76229439086 | | 1988 | 0.034319321576944 | | 1987 | 3.5467198521248 | | 1986 | 4.1063094623835 | | 1985 | 4.0255266419214 | | 1984 | 3.6211669781618 | | 1983 | 3.1539406915747 | | 1982 | 0.28379533940521 | | 1981 | 0.72230659744916 | | 1980 | 8.5603855246733 | | 1979 | 12.406977320998 | | 1978 | 7.1167033036157 | | 1977 | 2.6352944376945 | | 1976 | 2.4642490155397 | | 1975 | 2.4531904465915 | | 1974 | 2.5402686649413 | | 1973 | 2.6225359344812 | | 1972 | 2.5808863125798 | | 1971 | 2.5842337706357 | | 1970 | 2.6056396554237 | | 1969 | 2.5946683187612 | | 1968 | 2.5988888666443 | | 1967 | 2.5940522129798 | | 1966 | 2.6000149434979 | | 1965 | 2.6044137391714 | | 1964 | 2.5907624241122 | | 1963 | 2.5909380364918 | | 1962 | 2.5858906622001 | | 1961 | 2.5766530102416 |
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