Year	Years
1970	2.56
1971	2.63
1972	2.7
1973	2.78
1974	2.85
1975	2.93
1976	3.01
1977	3.09
1978	3.17
1979	3.26
1980	3.35
1981	3.44
1982	3.54
1983	3.63
1984	3.73
1985	3.83
1986	3.94
1987	4.05
1988	4.16
1989	4.28
1990	4.4
1991	4.52
1992	4.65
1993	4.79
1994	4.92
1995	5.07
1996	5.22
1997	5.38
1998	5.54
1999	5.69
2000	5.85
2001	6
2002	6.16
2003	6.31
2004	6.47
2005	6.62
2006	6.77
2007	6.92
2008	7.07
2009	7.22
2010	7.37
2011	7.52
2012	7.67
2013	7.82
2014	7.97
2015	8.11