| Year | Value | | 2021 | 7.4618100416618 | | 2020 | 7.4685793248667 | | 2019 | 7.4819859836147 | | 2018 | 7.4756514872335 | | 2017 | 7.4690708081074 | | 2016 | 7.4857688131355 | | 2015 | 7.4805040966076 | | 2014 | 7.4602652275494 | | 2013 | 7.4319655138372 | | 2012 | 7.519497186482 | | 2011 | 7.5244331830597 | | 2010 | 7.5416255347154 | | 2009 | 7.5567620927937 | | 2008 | 7.5564330371833 | | 2007 | 7.5467789141373 | | 2006 | 7.5656834697971 | | 2005 | 7.4651170425818 | | 2004 | 7.3968285235891 | | 2003 | 7.3738468104665 | | 2002 | 7.34101579172 | | 2001 | 7.2950523654749 | | 2000 | 7.2819199579763 | | 1999 | 7.2274204668571 | | 1998 | 7.1965593092354 | | 1997 | 7.0586690305 | | 1996 | 7.2819199579763 | | 1995 | 7.3672806067172 | | 1994 | 7.9185134114712 | | 1993 | 7.825929938606 | | 1992 | 7.900784661348 | | 1991 | 7.9608654256542 | | 1990 | 7.8564627860403 | | 1989 | 7.8328244525428 | | 1988 | 7.7914573689222 | | 1987 | 7.90570931416 | | 1986 | 7.8577760267901 | | 1985 | 7.9070225549099 | | 1984 | 7.9638202173413 | | 1983 | 8.0449128336452 | | 1982 | 8.1673725335697 | | 1981 | 8.2625824879346 | | 1980 | 8.3259463541154 | | 1979 | 8.4293640631669 | | 1978 | 8.4871466561607 | | 1977 | 8.5281854295939 | | 1976 | 8.5751337864014 | | 1975 | 8.5813716799632 | | 1974 | 8.5537936242162 | | 1973 | 8.4772973505368 | | 1972 | 8.5810433697758 | | 1971 | 8.8765225384944 | | 1970 | 8.9438261269247 | | 1969 | 9.0794182343478 | | 1968 | 9.4753603204307 | | 1967 | 9.4336649266227 | | 1966 | 9.3877015003776 | | 1965 | 9.3289339768213 | | 1964 | 9.2767326570143 | | 1963 | 9.2140254112085 | | 1962 | 9.143767031091 | | 1961 | 9.0800748547227 |
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