Year	Years
1970	3.8
1971	3.9
1972	4
1973	4.1
1974	4.21
1975	4.32
1976	4.44
1977	4.55
1978	4.67
1979	4.79
1980	4.92
1981	5.04
1982	5.17
1983	5.31
1984	5.44
1985	5.58
1986	5.72
1987	5.87
1988	6.01
1989	6.16
1990	6.31
1991	6.47
1992	6.62
1993	6.78
1994	6.93
1995	7.09
1996	7.23
1997	7.37
1998	7.51
1999	7.65
2000	7.79
2001	7.93
2002	8.07
2003	8.21
2004	8.35
2005	8.49
2006	8.63
2007	8.77
2008	8.9
2009	9.04
2010	9.18
2011	9.32
2012	9.46
2013	9.6
2014	9.74
2015	9.88