Year	Years
1970	3.21
1971	3.28
1972	3.36
1973	3.44
1974	3.52
1975	3.6
1976	3.69
1977	3.77
1978	3.86
1979	3.94
1980	4.03
1981	4.12
1982	4.22
1983	4.31
1984	4.4
1985	4.5
1986	4.6
1987	4.7
1988	4.8
1989	4.9
1990	5.01
1991	5.11
1992	5.22
1993	5.33
1994	5.44
1995	5.55
1996	5.66
1997	5.77
1998	5.88
1999	6
2000	6.12
2001	6.23
2002	6.35
2003	6.47
2004	6.6
2005	6.72
2006	6.84
2007	6.97
2008	7.09
2009	7.22
2010	7.35
2011	7.48
2012	7.61
2013	7.74
2014	7.87
2015	8.01