| Year | Value | | 2022 | 2.9986276396955 | | 2021 | 3.211427903809 | | 2020 | 3.3328087452617 | | 2019 | 3.395090564274 | | 2018 | 3.4012779585003 | | 2017 | 3.4964684655321 | | 2016 | 3.3349947470103 | | 2015 | 3.0913091564717 | | 2014 | 2.9689860734774 | | 2013 | 2.8767478792897 | | 2012 | 2.8937078691931 | | 2011 | 2.9071115803738 | | 2010 | 2.9151745444787 | | 2009 | 2.9156563947553 | | 2008 | 2.9269228964889 | | 2007 | 2.9335409922777 | | 2006 | 2.915021970834 | | 2005 | 2.9062316765633 | | 2004 | 2.9584807067091 | | 2003 | 3.1202867211522 | | 2002 | 3.1081397994416 | | 2001 | 3.0447894260234 | | 2000 | 3.1353556736977 | | 1999 | 3.2374677936436 | | 1998 | 2.9754713568421 | | 1997 | 2.9096131483314 | | 1996 | 2.7129521405818 | | 1995 | 2.7243365802957 | | 1994 | 3.3450797464942 | | 1993 | 3.4551630480942 | | 1992 | 3.4083067877244 | | 1991 | 3.2739416693198 | | 1990 | 3.2248018154563 | | 1989 | 3.2177855567696 | | 1988 | 3.1737158166777 | | 1987 | 3.0987422185555 | | 1986 | 2.9347325229464 | | 1985 | 2.839135980452 | | 1984 | 2.6972458577829 | | 1983 | 2.4824655110279 | | 1982 | 2.3066063994628 | | 1981 | 2.0824376720328 | | 1980 | 2.072783354419 | | 1979 | 2.4075044557169 | | 1978 | 2.6545586107625 | | 1977 | 2.6545314661956 | | 1976 | 2.6437066379177 | | 1975 | 2.6105779927555 | | 1974 | 2.5036064358634 | | 1973 | 2.3652155315154 | | 1972 | 2.5043280645868 | | 1971 | 2.858056674084 | | 1970 | 3.0576088372946 | | 1969 | 3.1485891478377 | | 1968 | 3.1740867014152 | | 1967 | 3.1215235118847 | | 1966 | 3.0747385898388 | | 1965 | 3.0392574053467 | | 1964 | 3.0074023818334 | | 1963 | 2.9573260270745 | | 1962 | 2.8968407519541 | | 1961 | 2.8559309555398 |
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