| Year | Value | | 2022 | 2.1446284210602 | | 2021 | 2.137789700681 | | 2020 | 2.3104473969763 | | 2019 | 2.4262079294499 | | 2018 | 2.4863597889306 | | 2017 | 2.5643206780196 | | 2016 | 2.6215373231585 | | 2015 | 2.7129548982116 | | 2014 | 2.8110724290721 | | 2013 | 2.8592373147357 | | 2012 | 2.8930274863714 | | 2011 | 2.914067083312 | | 2010 | 2.9294907051517 | | 2009 | 2.9588552672734 | | 2008 | 2.9911209419061 | | 2007 | 3.0276814194877 | | 2006 | 2.983229809708 | | 2005 | 2.793085570385 | | 2004 | 2.6618270232095 | | 2003 | 2.6508616845478 | | 2002 | 2.6658342590923 | | 2001 | 2.7257583971624 | | 2000 | 2.7987809098691 | | 1999 | 2.8354695574762 | | 1998 | 2.877490206929 | | 1997 | 2.9320660281261 | | 1996 | 3.1238693437747 | | 1995 | 3.476210112677 | | 1994 | 3.701152389686 | | 1993 | 3.7682347562304 | | 1992 | 3.7980362834984 | | 1991 | 3.8193966901122 | | 1990 | 3.8313807117685 | | 1989 | 3.841578327318 | | 1988 | 3.8821388281034 | | 1987 | 3.8507718679115 | | 1986 | 3.8091806911092 | | 1985 | 3.7971463664297 | | 1984 | 3.7227569551859 | | 1983 | 3.6314756660107 | | 1982 | 3.5381246031762 | | 1981 | 3.4610720107486 | | 1980 | 3.3697566537759 | | 1979 | 3.2502704225428 | | 1978 | 3.1496017822883 | | 1977 | 3.079905647423 | | 1976 | 3.0010824929336 | | 1975 | 2.9121162512746 | | 1974 | 2.8238857363608 | | 1973 | 2.7595972681279 | | 1972 | 2.6917156373721 | | 1971 | 2.6045828450213 | | 1970 | 2.5098398364617 | | 1969 | 2.3970575790427 | | 1968 | 2.2958310634131 | | 1967 | 2.2163352412265 | | 1966 | 2.1278572391963 | | 1965 | 2.0461979208041 | | 1964 | 1.9200522248561 | | 1963 | 1.8394669712467 | | 1962 | 1.8722914375551 | | 1961 | 1.8627371586163 |
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