| Year | Value | | 2022 | 2.1799711581794 | | 2021 | 2.0977641279856 | | 2020 | 2.1130542586751 | | 2019 | 2.0873510590356 | | 2018 | 2.0152167643083 | | 2017 | 1.6051086074809 | | 2016 | 1.5156791347454 | | 2015 | 1.4062000901307 | | 2014 | 1.3925723298783 | | 2013 | 1.3252248760703 | | 2012 | 1.435167192429 | | 2011 | 1.4486913023885 | | 2010 | 1.3871518702118 | | 2009 | 1.4574583145561 | | 2008 | 1.5248922938261 | | 2007 | 1.5703740423614 | | 2006 | 1.4234186570527 | | 2005 | 1.6517863902659 | | 2004 | 1.796609283461 | | 2003 | 1.889347453808 | | 2002 | 1.8847363677332 | | 2001 | 2.2637232987832 | | 2000 | 2.5398828301036 | | 1999 | 2.2857142857143 | | 1998 | 2.3470031545741 | | 1997 | 2.2884182063993 | | 1996 | 2.1928796755295 | | 1995 | 2.1622352410996 | | 1994 | 2.5525011266336 | | 1993 | 2.6345200540784 | | 1992 | 2.7670121676431 | | 1991 | 2.7381703470031 | | 1990 | 2.9788192879675 | | 1989 | 3.1293375394322 | | 1988 | 3.2798557908968 | | 1987 | 3.4465975664714 | | 1986 | 3.4294727354664 | | 1985 | 3.4159531320415 | | 1984 | 3.4015322217215 | | 1983 | 3.3880126182965 | | 1982 | 3.3735917079766 | | 1981 | 3.3600721045516 | | 1980 | 3.3456511942316 | | 1979 | 3.3321315908067 | | 1978 | 3.3177106804867 | | 1977 | 3.3041910770617 | | 1976 | 3.2897701667418 | | 1975 | 3.2762505633168 |
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