Year	Years
1970	3.65
1971	3.74
1972	3.83
1973	3.93
1974	4.02
1975	4.12
1976	4.22
1977	4.32
1978	4.42
1979	4.52
1980	4.63
1981	4.73
1982	4.84
1983	4.95
1984	5.06
1985	5.18
1986	5.29
1987	5.41
1988	5.53
1989	5.64
1990	5.77
1991	5.89
1992	6.01
1993	6.13
1994	6.26
1995	6.38
1996	6.51
1997	6.63
1998	6.76
1999	6.88
2000	7.01
2001	7.14
2002	7.26
2003	7.39
2004	7.52
2005	7.65
2006	7.78
2007	7.92
2008	8.05
2009	8.18
2010	8.32
2011	8.45
2012	8.59
2013	8.72
2014	8.86
2015	8.99