Year Years 1970 4.95 1971 5.07 1972 5.18 1973 5.3 1974 5.41 1975 5.53 1976 5.65 1977 5.76 1978 5.88 1979 6 1980 6.11 1981 6.23 1982 6.35 1983 6.46 1984 6.58 1985 6.69 1986 6.8 1987 6.91 1988 7.02 1989 7.13 1990 7.23 1991 7.33 1992 7.43 1993 7.53 1994 7.62 1995 7.71 1996 7.83 1997 7.95 1998 8.08 1999 8.2 2000 8.34 2001 8.47 2002 8.61 2003 8.75 2004 8.89 2005 9.03 2006 9.18 2007 9.33 2008 9.48 2009 9.63 2010 9.79 2011 9.94 2012 10.1 2013 10.25 2014 10.41 2015 10.57