Year	Years
1970	3.77
1971	3.85
1972	3.94
1973	4.03
1974	4.13
1975	4.22
1976	4.32
1977	4.41
1978	4.51
1979	4.61
1980	4.71
1981	4.82
1982	4.92
1983	5.03
1984	5.14
1985	5.25
1986	5.36
1987	5.47
1988	5.58
1989	5.7
1990	5.81
1991	5.93
1992	6.05
1993	6.17
1994	6.29
1995	6.41
1996	6.53
1997	6.66
1998	6.78
1999	6.9
2000	7.03
2001	7.15
2002	7.28
2003	7.41
2004	7.54
2005	7.67
2006	7.8
2007	7.93
2008	8.06
2009	8.19
2010	8.32
2011	8.46
2012	8.59
2013	8.72
2014	8.86
2015	9