| Year | Value | | 2022 | 2.4252271169662 | | 2021 | 2.4714466286161 | | 2020 | 2.8125275618297 | | 2019 | 2.5486221739175 | | 2018 | 2.4242845260442 | | 2017 | 2.5315113605679 | | 2016 | 2.5431515211454 | | 2015 | 2.4574500606016 | | 2014 | 2.5439817646226 | | 2013 | 2.5488248901417 | | 2012 | 2.6181663475856 | | 2011 | 2.7044822227697 | | 2010 | 2.8894632716464 | | 2009 | 3.1293847273747 | | 2008 | 2.6314607812411 | | 2007 | 2.4782465280356 | | 2006 | 2.6803815602276 | | 2005 | 2.9105875016448 | | 2004 | 2.8287522479811 | | 2003 | 2.6777769322275 | | 2002 | 2.8268668304584 | | 2001 | 2.9244355462896 | | 2000 | 2.9489299062257 | | 1999 | 2.9571956474341 | | 1998 | 2.7273114167938 | | 1997 | 2.6477324260172 | | 1996 | 2.4727682254674 | | 1995 | 2.5784835724607 | | 1994 | 2.664673727086 | | 1993 | 2.8235422456872 | | 1992 | 2.7048397056141 | | 1991 | 2.9099743276427 | | 1990 | 3.1462146102996 | | 1989 | 3.5341797276729 | | 1988 | 3.7286717827627 | | 1987 | 4.2313175270108 | | 1986 | 4.1064686806622 | | 1985 | 3.5690302776083 | | 1984 | 3.4168755943633 | | 1983 | 3.3230686597074 | | 1982 | 3.3451176655514 | | 1981 | 3.1862481221833 | | 1980 | 3.1337813297258 | | 1979 | 3.2613286578754 | | 1978 | 3.1263317016988 | | 1977 | 3.1627719371292 | | 1976 | 3.4778831428908 | | 1975 | 3.5345987811072 | | 1974 | 3.2028263244129 | | 1973 | 3.1586437671343 | | 1972 | 3.7179474612821 | | 1971 | 3.6524535235625 | | 1970 | 3.1851830398517 | | 1969 | 3.1408145580589 | | 1968 | 3.2455685846741 | | 1967 | 3.2239237214678 | | 1966 | 3.5723706459249 | | 1965 | 3.8714149139579 | | 1964 | 3.819236980218 | | 1963 | 4.0253060263654 | | 1962 | 2.7454004329004 | | 1961 | 2.0743597206054 | | 1960 | 2.004012345679 |
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