Year	Years
1970	4.03
1971	4.11
1972	4.2
1973	4.29
1974	4.38
1975	4.46
1976	4.55
1977	4.63
1978	4.72
1979	4.8
1980	4.88
1981	4.96
1982	5.04
1983	5.12
1984	5.2
1985	5.28
1986	5.35
1987	5.43
1988	5.5
1989	5.57
1990	5.65
1991	5.72
1992	5.79
1993	5.86
1994	5.92
1995	5.99
1996	6.1
1997	6.22
1998	6.33
1999	6.45
2000	6.57
2001	6.7
2002	6.83
2003	6.96
2004	7.1
2005	7.23
2006	7.37
2007	7.52
2008	7.66
2009	7.81
2010	7.96
2011	8.11
2012	8.26
2013	8.41
2014	8.57
2015	8.73