Year	Years
1970	3.06
1971	3.14
1972	3.22
1973	3.3
1974	3.38
1975	3.46
1976	3.55
1977	3.63
1978	3.72
1979	3.81
1980	3.91
1981	4
1982	4.1
1983	4.2
1984	4.3
1985	4.4
1986	4.5
1987	4.61
1988	4.72
1989	4.83
1990	4.94
1991	5.05
1992	5.17
1993	5.28
1994	5.4
1995	5.53
1996	5.65
1997	5.77
1998	5.9
1999	6.03
2000	6.15
2001	6.28
2002	6.41
2003	6.54
2004	6.67
2005	6.8
2006	6.94
2007	7.07
2008	7.2
2009	7.33
2010	7.47
2011	7.6
2012	7.74
2013	7.87
2014	8.01
2015	8.14