| Year | Value | | 2023 | 1.9232743367271 | | 2022 | 1.9383768701044 | | 2021 | 1.9763198807835 | | 2020 | 1.9619937976261 | | 2019 | 1.9238570270033 | | 2018 | 1.9527415361622 | | 2017 | 1.9925779873854 | | 2016 | 2.0511157819335 | | 2015 | 2.1109333844685 | | 2014 | 2.1754149604709 | | 2013 | 2.1138495729419 | | 2012 | 2.011498627278 | | 2011 | 1.9987179753105 | | 2010 | 1.9879076840121 | | 2009 | 1.988653182347 | | 2008 | 1.9868511460497 | | 2007 | 1.9930592275643 | | 2006 | 2.0170547631074 | | 2005 | 2.0408401981718 | | 2004 | 1.8959248538476 | | 2003 | 1.9270194361842 | | 2002 | 2.1160802344277 | | 2001 | 2.0567511683103 | | 2000 | 1.9198944454577 | | 1999 | 1.7821385766389 | | 1998 | 1.7220698621757 | | 1997 | 1.722187319954 | | 1996 | 1.7219906691372 | | 1995 | 1.7361918459321 | | 1994 | 1.7702465997598 | | 1993 | 1.8255823656646 | | 1992 | 2.0761735852254 | | 1991 | 2.4352646326938 | | 1990 | 2.6190429635189 | | 1989 | 2.671224232856 | | 1988 | 2.7036014093439 | | 1987 | 2.7469226640126 | | 1986 | 2.7845947095249 | | 1985 | 2.7978728086917 | | 1984 | 2.7936892769661 | | 1983 | 2.7871462246721 | | 1982 | 2.7700022749521 | | 1981 | 2.8204928163414 | | 1980 | 2.9843900662622 | | 1979 | 3.1306441087523 | | 1978 | 3.233780425977 | | 1977 | 3.3190016898609 | | 1976 | 3.3651484493448 | | 1975 | 3.3999601949659 |
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