Year	Years
1970	3.07
1971	3.15
1972	3.23
1973	3.3
1974	3.39
1975	3.47
1976	3.55
1977	3.64
1978	3.72
1979	3.81
1980	3.9
1981	3.99
1982	4.08
1983	4.18
1984	4.27
1985	4.37
1986	4.47
1987	4.57
1988	4.67
1989	4.77
1990	4.87
1991	4.97
1992	5.08
1993	5.19
1994	5.29
1995	5.4
1996	5.5
1997	5.6
1998	5.71
1999	5.81
2000	5.92
2001	6.02
2002	6.13
2003	6.25
2004	6.36
2005	6.48
2006	6.59
2007	6.71
2008	6.83
2009	6.96
2010	7.08
2011	7.21
2012	7.33
2013	7.46
2014	7.59
2015	7.72