Year % 1963 27.3 1964 27.8 1965 27.2 1966 27.2 1967 26.8 1968 26.5 1969 26.3 1970 26.6 1971 27 1972 26.5 1973 26.2 1974 25.7 1975 25.8 1976 25.7 1977 26 1978 25.2 1979 25.3 1980 25.2 1981 25.7 1982 26.3 1983 26.6 1984 26.6 1985 27 1986 26.8 1987 26.5 1988 27.1 1989 27.7 1990 27.6 1991 27.5 1992 27.7 1993 29.7 1994 29.6 1995 29.7 1996 30 1997 30.4 1998 29.7 1999 29.9 2000 30.3 2001 30.6 2002 30.1 2003 30.2 2004 30.1 2005 30.8 2006 31.2 2007 30.4 2008 30.3 2009 30.4 2010 29.3 2011 30.2 2012 30.2 2013 30 2014 30.7 2015 30.5 2016 30.4 2017 30.3 2018 30.7 2019 30.8 2020 29.5 2021 30.1