Year	Years
1970	3.51
1971	3.58
1972	3.66
1973	3.74
1974	3.82
1975	3.9
1976	3.98
1977	4.06
1978	4.14
1979	4.22
1980	4.31
1981	4.39
1982	4.47
1983	4.56
1984	4.64
1985	4.73
1986	4.81
1987	4.89
1988	4.98
1989	5.06
1990	5.14
1991	5.22
1992	5.3
1993	5.38
1994	5.46
1995	5.53
1996	5.6
1997	5.68
1998	5.76
1999	5.85
2000	5.94
2001	6.03
2002	6.12
2003	6.22
2004	6.32
2005	6.42
2006	6.52
2007	6.63
2008	6.74
2009	6.85
2010	6.96
2011	7.07
2012	7.19
2013	7.3
2014	7.42
2015	7.54