Year	Years
1970	3.65
1971	3.73
1972	3.81
1973	3.9
1974	3.99
1975	4.08
1976	4.17
1977	4.26
1978	4.36
1979	4.45
1980	4.55
1981	4.65
1982	4.75
1983	4.85
1984	4.95
1985	5.06
1986	5.16
1987	5.27
1988	5.38
1989	5.49
1990	5.6
1991	5.71
1992	5.83
1993	5.94
1994	6.06
1995	6.17
1996	6.29
1997	6.4
1998	6.52
1999	6.64
2000	6.76
2001	6.88
2002	7
2003	7.12
2004	7.25
2005	7.38
2006	7.5
2007	7.63
2008	7.76
2009	7.89
2010	8.02
2011	8.15
2012	8.29
2013	8.42
2014	8.55
2015	8.69