1977 18.0 20.7 1978 18.0 20.7 1979 18.5 20.1 1980 19.0 21.7 1981 19.6 22.2 1982 19.2 23.1 1983 17.5 23.5 1984 17.3 22.2 1985 17.7 22.8 1986 17.5 22.5 1987 18.4 21.6 1988 18.2 21.3 1989 18.4 21.2 1990 18.0 21.9 1991 17.8 22.3 1992 17.5 22.1 1993 17.5 21.4 1994 18.0 21.0 1995 18.4 20.6 1996 18.8 20.2 1997 19.2 19.5 1998 19.9 19.1 1999 19.8 18.5 2000 20.6 18.2 2001 19.5 18.2 2002 17.6 19.1 2003 16.2 19.7 2004 16.1 19.6 2005 17.3 19.9 2006 18.2 20.1 2007 18.5 19.6 2008 17.5 20.7 2009 14.8 24.7 2010 14.8 25.4 2011 16.8 25.1 2012 18.1 23.2 2013 18.6 22.8 2014 19.0 22.9 2015 18.9 22.9