Year	Years
1970	3.24
1971	3.32
1972	3.4
1973	3.48
1974	3.56
1975	3.64
1976	3.73
1977	3.82
1978	3.9
1979	3.99
1980	4.09
1981	4.18
1982	4.27
1983	4.37
1984	4.47
1985	4.57
1986	4.67
1987	4.77
1988	4.87
1989	4.98
1990	5.08
1991	5.19
1992	5.3
1993	5.41
1994	5.53
1995	5.64
1996	5.75
1997	5.87
1998	5.99
1999	6.11
2000	6.23
2001	6.35
2002	6.47
2003	6.59
2004	6.72
2005	6.84
2006	6.97
2007	7.09
2008	7.22
2009	7.35
2010	7.48
2011	7.61
2012	7.74
2013	7.88
2014	8.01
2015	8.14