Year	Years
1970	3.11
1971	3.18
1972	3.26
1973	3.33
1974	3.41
1975	3.49
1976	3.57
1977	3.66
1978	3.74
1979	3.83
1980	3.92
1981	4.01
1982	4.1
1983	4.19
1984	4.29
1985	4.38
1986	4.48
1987	4.58
1988	4.68
1989	4.78
1990	4.89
1991	4.99
1992	5.1
1993	5.21
1994	5.32
1995	5.43
1996	5.55
1997	5.67
1998	5.78
1999	5.9
2000	6.03
2001	6.15
2002	6.27
2003	6.4
2004	6.52
2005	6.65
2006	6.78
2007	6.91
2008	7.04
2009	7.17
2010	7.3
2011	7.44
2012	7.57
2013	7.71
2014	7.84
2015	7.98