Year	Years
1970	3.04
1971	3.13
1972	3.22
1973	3.31
1974	3.4
1975	3.5
1976	3.59
1977	3.69
1978	3.79
1979	3.9
1980	4
1981	4.11
1982	4.21
1983	4.32
1984	4.43
1985	4.55
1986	4.66
1987	4.78
1988	4.89
1989	5.01
1990	5.13
1991	5.25
1992	5.37
1993	5.5
1994	5.62
1995	5.75
1996	5.88
1997	6.01
1998	6.14
1999	6.27
2000	6.41
2001	6.55
2002	6.69
2003	6.83
2004	6.98
2005	7.13
2006	7.28
2007	7.43
2008	7.58
2009	7.73
2010	7.89
2011	8.05
2012	8.21
2013	8.37
2014	8.53
2015	8.7