Year	Years
1970	3.21
1971	3.3
1972	3.4
1973	3.49
1974	3.59
1975	3.69
1976	3.79
1977	3.89
1978	4
1979	4.1
1980	4.21
1981	4.32
1982	4.44
1983	4.55
1984	4.67
1985	4.78
1986	4.9
1987	5.02
1988	5.15
1989	5.27
1990	5.4
1991	5.52
1992	5.65
1993	5.78
1994	5.91
1995	6.04
1996	6.18
1997	6.31
1998	6.45
1999	6.58
2000	6.72
2001	6.87
2002	7.01
2003	7.16
2004	7.31
2005	7.46
2006	7.61
2007	7.76
2008	7.92
2009	8.07
2010	8.23
2011	8.39
2012	8.55
2013	8.71
2014	8.88
2015	9.04