Year	Years
1970	3.77
1971	3.85
1972	3.94
1973	4.02
1974	4.11
1975	4.2
1976	4.29
1977	4.38
1978	4.47
1979	4.56
1980	4.66
1981	4.75
1982	4.85
1983	4.95
1984	5.05
1985	5.15
1986	5.26
1987	5.36
1988	5.47
1989	5.58
1990	5.7
1991	5.81
1992	5.93
1993	6.05
1994	6.18
1995	6.31
1996	6.44
1997	6.58
1998	6.71
1999	6.84
2000	6.98
2001	7.11
2002	7.25
2003	7.39
2004	7.52
2005	7.66
2006	7.79
2007	7.93
2008	8.07
2009	8.21
2010	8.34
2011	8.48
2012	8.62
2013	8.76
2014	8.9
2015	9.04