Year	Years
1970	3.7
1971	3.79
1972	3.89
1973	3.99
1974	4.08
1975	4.19
1976	4.29
1977	4.39
1978	4.5
1979	4.6
1980	4.71
1981	4.82
1982	4.93
1983	5.04
1984	5.16
1985	5.27
1986	5.39
1987	5.51
1988	5.63
1989	5.75
1990	5.87
1991	6
1992	6.13
1993	6.25
1994	6.38
1995	6.52
1996	6.66
1997	6.81
1998	6.96
1999	7.11
2000	7.26
2001	7.41
2002	7.57
2003	7.73
2004	7.89
2005	8.05
2006	8.21
2007	8.38
2008	8.54
2009	8.71
2010	8.88
2011	9.05
2012	9.22
2013	9.39
2014	9.56
2015	9.73