Year % 1968 22.3 1969 23 1970 23 1971 23.8 1972 22.9 1973 23.3 1974 23.5 1975 22.6 1976 22.2 1977 22 1978 21.9 1979 22 1980 22.9 1981 23.6 1982 23.7 1983 23.6 1984 23.2 1985 23.8 1986 25.3 1987 26.7 1988 26.9 1989 25.7 1990 27.5 1991 26.7 1992 26.6 1993 26.8 1994 27.8 1995 27.3 1996 27.2 1997 27.6 1998 28.5 1999 28.7 2000 31.1 2001 29.3 2002 27.7 2003 27.3 2004 27.3 2005 27.8 2006 28.3 2007 26.5 2008 27.6 2009 27.4 2010 26.4 2011 25.9 2012 25.6 2013 25.3 2014 25.6 2015 25.7 2016 25.5 2017 24.9 2018 25.8 2019 25.1 2020 25 2021 25