Year	Years
1970	3
1971	3.07
1972	3.15
1973	3.23
1974	3.32
1975	3.4
1976	3.49
1977	3.57
1978	3.66
1979	3.76
1980	3.85
1981	3.95
1982	4.05
1983	4.15
1984	4.25
1985	4.36
1986	4.47
1987	4.58
1988	4.69
1989	4.81
1990	4.92
1991	5.04
1992	5.17
1993	5.29
1994	5.42
1995	5.55
1996	5.67
1997	5.79
1998	5.92
1999	6.04
2000	6.17
2001	6.29
2002	6.42
2003	6.55
2004	6.68
2005	6.81
2006	6.94
2007	7.07
2008	7.2
2009	7.34
2010	7.47
2011	7.61
2012	7.74
2013	7.88
2014	8.01
2015	8.15