Year	Years
1970	2.75
1971	2.83
1972	2.91
1973	3
1974	3.09
1975	3.18
1976	3.27
1977	3.37
1978	3.46
1979	3.56
1980	3.67
1981	3.77
1982	3.88
1983	3.99
1984	4.1
1985	4.22
1986	4.33
1987	4.45
1988	4.58
1989	4.7
1990	4.83
1991	4.96
1992	5.09
1993	5.23
1994	5.37
1995	5.51
1996	5.66
1997	5.81
1998	5.96
1999	6.11
2000	6.26
2001	6.41
2002	6.57
2003	6.73
2004	6.89
2005	7.05
2006	7.21
2007	7.37
2008	7.53
2009	7.7
2010	7.86
2011	8.03
2012	8.2
2013	8.36
2014	8.53
2015	8.7