Year	Years
1970	3.47
1971	3.56
1972	3.64
1973	3.73
1974	3.82
1975	3.9
1976	4
1977	4.09
1978	4.18
1979	4.28
1980	4.37
1981	4.47
1982	4.57
1983	4.67
1984	4.78
1985	4.88
1986	4.99
1987	5.1
1988	5.21
1989	5.32
1990	5.43
1991	5.54
1992	5.66
1993	5.77
1994	5.89
1995	6.01
1996	6.12
1997	6.24
1998	6.36
1999	6.47
2000	6.59
2001	6.71
2002	6.84
2003	6.96
2004	7.09
2005	7.22
2006	7.34
2007	7.48
2008	7.61
2009	7.74
2010	7.88
2011	8.01
2012	8.15
2013	8.29
2014	8.43
2015	8.57