Year	Years
1970	3.34
1971	3.43
1972	3.52
1973	3.61
1974	3.7
1975	3.79
1976	3.89
1977	3.98
1978	4.08
1979	4.18
1980	4.27
1981	4.37
1982	4.47
1983	4.57
1984	4.67
1985	4.78
1986	4.88
1987	4.98
1988	5.08
1989	5.18
1990	5.28
1991	5.38
1992	5.48
1993	5.58
1994	5.68
1995	5.78
1996	5.89
1997	6
1998	6.12
1999	6.24
2000	6.36
2001	6.49
2002	6.62
2003	6.75
2004	6.88
2005	7.02
2006	7.16
2007	7.3
2008	7.44
2009	7.59
2010	7.74
2011	7.89
2012	8.04
2013	8.19
2014	8.35
2015	8.51