Year	Years
1970	2.71
1971	2.79
1972	2.87
1973	2.95
1974	3.03
1975	3.12
1976	3.21
1977	3.3
1978	3.39
1979	3.49
1980	3.59
1981	3.68
1982	3.79
1983	3.89
1984	3.99
1985	4.1
1986	4.21
1987	4.32
1988	4.44
1989	4.55
1990	4.67
1991	4.79
1992	4.91
1993	5.04
1994	5.17
1995	5.3
1996	5.43
1997	5.57
1998	5.7
1999	5.84
2000	5.99
2001	6.13
2002	6.28
2003	6.42
2004	6.57
2005	6.73
2006	6.88
2007	7.04
2008	7.19
2009	7.35
2010	7.51
2011	7.67
2012	7.84
2013	8
2014	8.17
2015	8.33