Year	Years
1970	3.74
1971	3.84
1972	3.95
1973	4.06
1974	4.17
1975	4.28
1976	4.4
1977	4.52
1978	4.64
1979	4.76
1980	4.89
1981	5.02
1982	5.14
1983	5.27
1984	5.41
1985	5.54
1986	5.68
1987	5.81
1988	5.95
1989	6.09
1990	6.23
1991	6.38
1992	6.52
1993	6.66
1994	6.81
1995	6.95
1996	7.1
1997	7.24
1998	7.39
1999	7.54
2000	7.69
2001	7.84
2002	7.99
2003	8.15
2004	8.3
2005	8.46
2006	8.62
2007	8.78
2008	8.94
2009	9.1
2010	9.26
2011	9.43
2012	9.59
2013	9.76
2014	9.92
2015	10.09