Year	Years
1970	3.4
1971	3.48
1972	3.57
1973	3.66
1974	3.75
1975	3.84
1976	3.93
1977	4.02
1978	4.12
1979	4.22
1980	4.32
1981	4.42
1982	4.52
1983	4.63
1984	4.73
1985	4.84
1986	4.96
1987	5.07
1988	5.19
1989	5.3
1990	5.42
1991	5.55
1992	5.67
1993	5.79
1994	5.92
1995	6.05
1996	6.18
1997	6.31
1998	6.44
1999	6.57
2000	6.71
2001	6.84
2002	6.97
2003	7.11
2004	7.24
2005	7.38
2006	7.51
2007	7.65
2008	7.79
2009	7.92
2010	8.06
2011	8.2
2012	8.34
2013	8.47
2014	8.61
2015	8.75