| Year | Value | | 2022 | 5.1676666180513 | | 2021 | 6.5579832202856 | | 2020 | 8.0302425053963 | | 2019 | 7.4392119076576 | | 2018 | 8.267601442385 | | 2017 | 8.4132499521215 | | 2016 | 10.56312622894 | | 2015 | 9.5711673032124 | | 2014 | 8.8604118139466 | | 2013 | 9.7472726010749 | | 2012 | 10.583203152896 | | 2011 | 6.45275108066 | | 2010 | 5.6488551649299 | | 2009 | 6.0725704751903 | | 2008 | 4.9602482238265 | | 2007 | 6.7175643169388 | | 2006 | 7.0990936186804 | | 2005 | 7.714034108533 | | 2004 | 7.8603795971782 | | 2003 | 7.9620349865159 | | 2002 | 8.1006656545783 | | 2001 | 8.1956403711877 | | 2000 | 7.0857087829274 | | 1999 | 7.5167882193274 | | 1998 | 8.2160921671518 | | 1997 | 9.3579111585516 | | 1996 | 9.4070782901792 | | 1995 | 10.967647723847 | | 1994 | 11.766452600004 | | 1993 | 11.525730701973 | | 1992 | 12.184092902795 | | 1991 | 11.06391029169 | | 1990 | 12.3909414645 | | 1989 | 12.5 | | 1988 | 13.699798418359 | | 1987 | 13.202314926453 | | 1986 | 15.866577591143 | | 1985 | 15.561466050242 | | 1984 | 16.893041675691 | | 1983 | 17.159369885434 | | 1982 | 16.666029220531 | | 1981 | 15.713425647201 | | 1980 | 14.909881307087 | | 1979 | 15.640747344755 | | 1978 | 20.949941915725 | | 1977 | 18.781685678074 | | 1976 | 23.009725206378 | | 1975 | 24.958574979288 | | 1974 | 15.527704485488 | | 1973 | 18.595041322314 | | 1972 | 14.861505681818 | | 1971 | 9.652278177458 |
|