Year	Years
1970	3.89
1971	3.99
1972	4.1
1973	4.21
1974	4.33
1975	4.44
1976	4.56
1977	4.68
1978	4.8
1979	4.93
1980	5.05
1981	5.18
1982	5.31
1983	5.44
1984	5.58
1985	5.71
1986	5.85
1987	5.99
1988	6.13
1989	6.28
1990	6.42
1991	6.57
1992	6.72
1993	6.87
1994	7.02
1995	7.17
1996	7.33
1997	7.48
1998	7.64
1999	7.8
2000	7.96
2001	8.13
2002	8.29
2003	8.45
2004	8.62
2005	8.78
2006	8.95
2007	9.12
2008	9.28
2009	9.45
2010	9.62
2011	9.79
2012	9.96
2013	10.12
2014	10.29
2015	10.46