Year	Years
1970	3.15
1971	3.22
1972	3.3
1973	3.38
1974	3.47
1975	3.55
1976	3.64
1977	3.73
1978	3.82
1979	3.91
1980	4
1981	4.1
1982	4.2
1983	4.3
1984	4.41
1985	4.51
1986	4.62
1987	4.73
1988	4.85
1989	4.96
1990	5.08
1991	5.2
1992	5.33
1993	5.46
1994	5.59
1995	5.72
1996	5.86
1997	5.99
1998	6.13
1999	6.27
2000	6.41
2001	6.55
2002	6.69
2003	6.83
2004	6.97
2005	7.11
2006	7.25
2007	7.39
2008	7.53
2009	7.67
2010	7.81
2011	7.95
2012	8.1
2013	8.24
2014	8.38
2015	8.52