Year	Years
1970	3.97
1971	4.05
1972	4.13
1973	4.21
1974	4.29
1975	4.38
1976	4.46
1977	4.54
1978	4.63
1979	4.71
1980	4.8
1981	4.88
1982	4.97
1983	5.05
1984	5.14
1985	5.22
1986	5.31
1987	5.39
1988	5.48
1989	5.56
1990	5.65
1991	5.73
1992	5.81
1993	5.9
1994	5.98
1995	6.06
1996	6.17
1997	6.28
1998	6.39
1999	6.5
2000	6.62
2001	6.73
2002	6.85
2003	6.97
2004	7.09
2005	7.22
2006	7.34
2007	7.47
2008	7.59
2009	7.72
2010	7.85
2011	7.98
2012	8.12
2013	8.25
2014	8.38
2015	8.52