Year	Years
1970	3.95
1971	4.04
1972	4.14
1973	4.23
1974	4.32
1975	4.42
1976	4.52
1977	4.62
1978	4.72
1979	4.82
1980	4.92
1981	5.03
1982	5.13
1983	5.24
1984	5.35
1985	5.46
1986	5.57
1987	5.69
1988	5.8
1989	5.92
1990	6.04
1991	6.16
1992	6.28
1993	6.4
1994	6.52
1995	6.65
1996	6.77
1997	6.9
1998	7.02
1999	7.15
2000	7.28
2001	7.41
2002	7.54
2003	7.67
2004	7.81
2005	7.94
2006	8.08
2007	8.21
2008	8.35
2009	8.48
2010	8.62
2011	8.76
2012	8.9
2013	9.04
2014	9.18
2015	9.32