Year	Years
1970	3.22
1971	3.3
1972	3.38
1973	3.46
1974	3.55
1975	3.63
1976	3.72
1977	3.81
1978	3.9
1979	3.99
1980	4.09
1981	4.18
1982	4.28
1983	4.38
1984	4.49
1985	4.59
1986	4.7
1987	4.81
1988	4.92
1989	5.04
1990	5.15
1991	5.27
1992	5.4
1993	5.52
1994	5.65
1995	5.78
1996	5.92
1997	6.05
1998	6.19
1999	6.32
2000	6.46
2001	6.6
2002	6.73
2003	6.87
2004	7.01
2005	7.15
2006	7.29
2007	7.43
2008	7.57
2009	7.71
2010	7.85
2011	7.99
2012	8.13
2013	8.27
2014	8.42
2015	8.56