Year	Years
1970	3.13
1971	3.22
1972	3.32
1973	3.41
1974	3.51
1975	3.62
1976	3.72
1977	3.83
1978	3.94
1979	4.05
1980	4.17
1981	4.28
1982	4.4
1983	4.53
1984	4.65
1985	4.78
1986	4.91
1987	5.04
1988	5.18
1989	5.32
1990	5.46
1991	5.6
1992	5.75
1993	5.9
1994	6.05
1995	6.2
1996	6.36
1997	6.51
1998	6.67
1999	6.82
2000	6.98
2001	7.14
2002	7.3
2003	7.47
2004	7.63
2005	7.79
2006	7.96
2007	8.12
2008	8.29
2009	8.46
2010	8.62
2011	8.79
2012	8.96
2013	9.13
2014	9.29
2015	9.46