Year	Years
1970	3.23
1971	3.32
1972	3.41
1973	3.51
1974	3.61
1975	3.71
1976	3.82
1977	3.92
1978	4.03
1979	4.14
1980	4.26
1981	4.37
1982	4.49
1983	4.61
1984	4.74
1985	4.86
1986	4.99
1987	5.12
1988	5.25
1989	5.38
1990	5.52
1991	5.66
1992	5.8
1993	5.95
1994	6.09
1995	6.24
1996	6.39
1997	6.55
1998	6.7
1999	6.86
2000	7.02
2001	7.18
2002	7.34
2003	7.5
2004	7.66
2005	7.83
2006	7.99
2007	8.16
2008	8.33
2009	8.49
2010	8.66
2011	8.83
2012	9
2013	9.17
2014	9.34
2015	9.51