Year	Years
1970	3.19
1971	3.27
1972	3.36
1973	3.44
1974	3.53
1975	3.61
1976	3.7
1977	3.8
1978	3.89
1979	3.99
1980	4.08
1981	4.18
1982	4.29
1983	4.39
1984	4.5
1985	4.61
1986	4.72
1987	4.83
1988	4.94
1989	5.06
1990	5.18
1991	5.3
1992	5.42
1993	5.54
1994	5.66
1995	5.78
1996	5.89
1997	6
1998	6.12
1999	6.24
2000	6.36
2001	6.48
2002	6.6
2003	6.72
2004	6.84
2005	6.97
2006	7.1
2007	7.23
2008	7.35
2009	7.48
2010	7.62
2011	7.75
2012	7.88
2013	8.02
2014	8.15
2015	8.29