Year % 1968 36.7 1969 37.5 1970 37.6 1971 38.6 1972 37.8 1973 38.2 1974 38.2 1975 37.3 1976 37.1 1977 36.7 1978 36.6 1979 37 1980 37.8 1981 38.6 1982 38.8 1983 38.7 1984 38 1985 39.1 1986 40.2 1987 41.9 1988 42.2 1989 41.1 1990 42.9 1991 42.3 1992 42.3 1993 42.3 1994 43.2 1995 42.8 1996 42.6 1997 42.9 1998 43.7 1999 43.8 2000 45.8 2001 44.3 2002 42.7 2003 42.3 2004 42.4 2005 42.8 2006 43.3 2007 41.8 2008 42.7 2009 42.5 2010 41.5 2011 41 2012 40.9 2013 40.5 2014 40.8 2015 40.8 2016 40.6 2017 40.2 2018 41.1 2019 40.2 2020 40.3 2021 40