Year	Years
1970	3.63
1971	3.71
1972	3.8
1973	3.88
1974	3.96
1975	4.05
1976	4.13
1977	4.22
1978	4.3
1979	4.39
1980	4.48
1981	4.57
1982	4.65
1983	4.74
1984	4.83
1985	4.91
1986	5
1987	5.09
1988	5.17
1989	5.25
1990	5.34
1991	5.42
1992	5.5
1993	5.58
1994	5.66
1995	5.73
1996	5.81
1997	5.88
1998	5.96
1999	6.05
2000	6.13
2001	6.22
2002	6.31
2003	6.41
2004	6.5
2005	6.6
2006	6.7
2007	6.8
2008	6.91
2009	7.02
2010	7.13
2011	7.24
2012	7.35
2013	7.47
2014	7.59
2015	7.71