Year	Years
1970	2.69
1971	2.77
1972	2.85
1973	2.94
1974	3.02
1975	3.11
1976	3.2
1977	3.3
1978	3.39
1979	3.49
1980	3.59
1981	3.69
1982	3.8
1983	3.9
1984	4.01
1985	4.12
1986	4.24
1987	4.35
1988	4.47
1989	4.59
1990	4.72
1991	4.84
1992	4.97
1993	5.1
1994	5.23
1995	5.37
1996	5.51
1997	5.65
1998	5.79
1999	5.93
2000	6.08
2001	6.22
2002	6.37
2003	6.52
2004	6.67
2005	6.83
2006	6.98
2007	7.14
2008	7.3
2009	7.46
2010	7.62
2011	7.78
2012	7.94
2013	8.1
2014	8.27
2015	8.43