Year	Years
1970	3.4
1971	3.47
1972	3.55
1973	3.64
1974	3.72
1975	3.8
1976	3.89
1977	3.97
1978	4.06
1979	4.15
1980	4.24
1981	4.33
1982	4.42
1983	4.52
1984	4.61
1985	4.71
1986	4.81
1987	4.91
1988	5.01
1989	5.11
1990	5.21
1991	5.32
1992	5.42
1993	5.53
1994	5.64
1995	5.75
1996	5.86
1997	5.97
1998	6.08
1999	6.2
2000	6.32
2001	6.43
2002	6.55
2003	6.67
2004	6.79
2005	6.92
2006	7.04
2007	7.17
2008	7.29
2009	7.42
2010	7.55
2011	7.68
2012	7.81
2013	7.94
2014	8.08
2015	8.21