Year	Years
1970	2.73
1971	2.81
1972	2.89
1973	2.97
1974	3.06
1975	3.15
1976	3.23
1977	3.33
1978	3.42
1979	3.51
1980	3.61
1981	3.71
1982	3.81
1983	3.91
1984	4.02
1985	4.12
1986	4.23
1987	4.34
1988	4.45
1989	4.57
1990	4.68
1991	4.8
1992	4.92
1993	5.04
1994	5.16
1995	5.29
1996	5.42
1997	5.55
1998	5.69
1999	5.82
2000	5.96
2001	6.1
2002	6.24
2003	6.39
2004	6.53
2005	6.68
2006	6.83
2007	6.98
2008	7.14
2009	7.29
2010	7.45
2011	7.6
2012	7.76
2013	7.92
2014	8.08
2015	8.25