Year	Years
1970	3.4
1971	3.48
1972	3.55
1973	3.63
1974	3.72
1975	3.8
1976	3.88
1977	3.97
1978	4.05
1979	4.14
1980	4.23
1981	4.32
1982	4.41
1983	4.5
1984	4.59
1985	4.68
1986	4.78
1987	4.87
1988	4.97
1989	5.07
1990	5.16
1991	5.26
1992	5.36
1993	5.46
1994	5.56
1995	5.67
1996	5.79
1997	5.9
1998	6.02
1999	6.15
2000	6.27
2001	6.39
2002	6.52
2003	6.65
2004	6.78
2005	6.9
2006	7.04
2007	7.17
2008	7.3
2009	7.43
2010	7.56
2011	7.7
2012	7.83
2013	7.97
2014	8.1
2015	8.24