Year | Value | 2021 | 16.8 | 2020 | 15.8 | 2019 | 16.4 | 2018 | 17.1 | 2017 | 17.2 | 2016 | 17.4 | 2015 | 17 | 2014 | 17 | 2013 | 17.4 | 2012 | 18.6 | 2011 | 18.6 | 2010 | 16.8 | 2009 | 18.1 | 2008 | 17.8 | 2007 | 17.6 | 2006 | 17.3 | 2005 | 18.5 | 2004 | 17.4 | 2003 | 18.1 | 2002 | 17.7 | 2001 | 17.7 | 2000 | 18.6 | 1999 | 18.2 | 1998 | 17.9 | 1997 | 19.1 | 1996 | 21.1 | 1995 | 21 | 1994 | 22.9 | 1993 | 23.4 | 1992 | 22.6 | 1991 | 24.2 | 1990 | 23.1 | 1989 | 23 | 1988 | 23.8 | 1987 | 24.5 | 1986 | 26.2 | 1985 | 26.5 | 1984 | 26.9 | 1983 | 25.8 | 1982 | 24.1 | 1981 | 28.1 | 1980 | 28.9 | 1979 | 27.6 | 1978 | 28.9 | 1977 | 25.9 | 1976 | 27.1 | 1975 | 30.5 | 1974 | 32.1 | 1973 | 28.8 | 1972 | 30.8 | 1971 | 33.6 |
|