| Year | Value | | 2021 | 25.36523712819 | | 2020 | 24.802482328034 | | 2019 | 26.180352318897 | | 2018 | 26.945102586425 | | 2017 | 26.458512500121 | | 2016 | 26.669658017698 | | 2015 | 24.881376028375 | | 2014 | 24.869668672685 | | 2013 | 24.123272960109 | | 2012 | 24.268417906817 | | 2011 | 22.477694433385 | | 2010 | 20.386033572571 | | 2009 | 19.775680199895 | | 2008 | 20.213612614947 | | 2007 | 20.242842514179 | | 2006 | 19.979200737232 | | 2005 | 20.326505747434 | | 2004 | 19.134730056952 | | 2003 | 19.707676765013 | | 2002 | 21.266877230551 | | 2001 | 20.937772789461 | | 2000 | 22.470521838595 | | 1999 | 21.433046287726 | | 1998 | 20.528082159334 | | 1997 | 19.52190170556 | | 1996 | 18.63608852241 | | 1995 | 18.580841289389 | | 1990 | 15.121290569729 | | 1989 | 13.000297391438 | | 1988 | 15.09763674922 | | 1987 | 16.355457198283 | | 1986 | 15.592583690551 | | 1985 | 13.812118550396 | | 1984 | 13.970162145707 | | 1983 | 15.597849753132 | | 1982 | 14.934871124608 | | 1981 | 12.322829431391 | | 1980 | 13.77116860741 | | 1979 | 13.89605476996 | | 1978 | 14.286322498064 | | 1977 | 13.964909051354 | | 1976 | 14.099752367398 | | 1975 | 12.702940464911 | | 1974 | 12.636409583506 | | 1973 | 12.483640986376 | | 1972 | 12.90267933651 |
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