Year	Years
1970	3.16
1971	3.26
1972	3.37
1973	3.49
1974	3.6
1975	3.72
1976	3.85
1977	3.98
1978	4.11
1979	4.25
1980	4.39
1981	4.54
1982	4.69
1983	4.84
1984	5.01
1985	5.17
1986	5.34
1987	5.52
1988	5.71
1989	5.89
1990	6.09
1991	6.29
1992	6.49
1993	6.7
1994	6.92
1995	7.13
1996	7.29
1997	7.45
1998	7.62
1999	7.78
2000	7.94
2001	8.11
2002	8.27
2003	8.44
2004	8.61
2005	8.77
2006	8.94
2007	9.11
2008	9.28
2009	9.45
2010	9.61
2011	9.78
2012	9.95
2013	10.12
2014	10.29
2015	10.46