Year	Years
1970	3.1
1971	3.18
1972	3.25
1973	3.33
1974	3.41
1975	3.5
1976	3.58
1977	3.66
1978	3.75
1979	3.84
1980	3.93
1981	4.02
1982	4.11
1983	4.21
1984	4.31
1985	4.4
1986	4.5
1987	4.61
1988	4.71
1989	4.82
1990	4.93
1991	5.04
1992	5.15
1993	5.26
1994	5.38
1995	5.49
1996	5.62
1997	5.75
1998	5.88
1999	6.01
2000	6.14
2001	6.28
2002	6.41
2003	6.54
2004	6.68
2005	6.82
2006	6.95
2007	7.09
2008	7.23
2009	7.37
2010	7.5
2011	7.64
2012	7.78
2013	7.92
2014	8.06
2015	8.2