| Year | Value | | 2023 | 2.1933064378658 | | 2022 | 2.1933064378658 | | 2021 | 2.1933064378658 | | 2020 | 2.1933064378658 | | 2019 | 2.1933064378658 | | 2018 | 2.210352860958 | | 2017 | 2.202776672917 | | 2016 | 2.0754967138284 | | 2015 | 2.1546678788567 | | 2014 | 2.1849726310207 | | 2013 | 2.2747504593064 | | 2012 | 2.2804326003371 | | 2011 | 2.1989885788965 | | 2010 | 2.4452146902286 | | 2009 | 2.217929048999 | | 2008 | 2.3580885277573 | | 2007 | 2.5853741689869 | | 2006 | 2.6024205920791 | | 2005 | 2.4376385021876 | | 2004 | 2.509612288577 | | 2003 | 2.371346856829 | | 2002 | 2.6800765194992 | | 2001 | 2.7766729170218 | | 2000 | 2.9263026308313 | | 1999 | 2.9300907248518 | | 1998 | 2.962289524026 | | 1997 | 3.0437335454666 | | 1996 | 3.1194954258765 | | 1995 | 3.092978767733 | | 1994 | 3.0816144856715 | | 1993 | 3.0721442506203 | | 1992 | 2.6099967801201 | | 1991 | 2.8941038316571 | | 1990 | 2.8846335966059 | | 1989 | 2.6099967801201 | | 1988 | 2.6062086860996 | | 1987 | 2.6062086860996 | | 1986 | 2.5986324980586 | | 1985 | 2.5986324980586 | | 1984 | 2.5910563100176 | | 1983 | 2.5910563100176 | | 1982 | 2.5910563100176 | | 1981 | 2.5910563100176 | | 1980 | 2.5872682159971 | | 1979 | 2.5872682159971 | | 1978 | 2.5853741689869 | | 1977 | 2.5815860749664 | | 1976 | 2.5815860749664 |
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