Year	Years
1970	3.48
1971	3.58
1972	3.67
1973	3.77
1974	3.87
1975	3.97
1976	4.07
1977	4.17
1978	4.27
1979	4.37
1980	4.47
1981	4.57
1982	4.68
1983	4.78
1984	4.88
1985	4.97
1986	5.07
1987	5.17
1988	5.26
1989	5.35
1990	5.44
1991	5.53
1992	5.61
1993	5.7
1994	5.77
1995	5.85
1996	5.93
1997	6.01
1998	6.1
1999	6.19
2000	6.29
2001	6.39
2002	6.49
2003	6.6
2004	6.71
2005	6.82
2006	6.94
2007	7.06
2008	7.18
2009	7.31
2010	7.44
2011	7.57
2012	7.7
2013	7.84
2014	7.98
2015	8.13