Year	Years
1970	2.4
1971	2.48
1972	2.55
1973	2.63
1974	2.71
1975	2.79
1976	2.87
1977	2.95
1978	3.04
1979	3.13
1980	3.22
1981	3.31
1982	3.4
1983	3.5
1984	3.6
1985	3.7
1986	3.8
1987	3.91
1988	4.01
1989	4.12
1990	4.24
1991	4.35
1992	4.47
1993	4.59
1994	4.71
1995	4.83
1996	4.96
1997	5.09
1998	5.22
1999	5.35
2000	5.49
2001	5.62
2002	5.76
2003	5.91
2004	6.05
2005	6.19
2006	6.34
2007	6.49
2008	6.64
2009	6.79
2010	6.95
2011	7.1
2012	7.26
2013	7.42
2014	7.57
2015	7.74