Year	Years
1970	2.47
1971	2.53
1972	2.61
1973	2.68
1974	2.76
1975	2.83
1976	2.91
1977	3
1978	3.08
1979	3.17
1980	3.27
1981	3.36
1982	3.46
1983	3.56
1984	3.67
1985	3.78
1986	3.9
1987	4.02
1988	4.15
1989	4.28
1990	4.42
1991	4.57
1992	4.72
1993	4.88
1994	5.05
1995	5.24
1996	5.4
1997	5.56
1998	5.71
1999	5.87
2000	6.02
2001	6.18
2002	6.33
2003	6.48
2004	6.64
2005	6.79
2006	6.94
2007	7.09
2008	7.24
2009	7.39
2010	7.54
2011	7.69
2012	7.84
2013	7.98
2014	8.13
2015	8.28