Year	Years
1970	3.23
1971	3.29
1972	3.35
1973	3.41
1974	3.46
1975	3.52
1976	3.58
1977	3.63
1978	3.69
1979	3.74
1980	3.79
1981	3.85
1982	3.9
1983	3.95
1984	4
1985	4.04
1986	4.09
1987	4.14
1988	4.18
1989	4.23
1990	4.27
1991	4.31
1992	4.35
1993	4.39
1994	4.43
1995	4.47
1996	4.54
1997	4.61
1998	4.69
1999	4.76
2000	4.84
2001	4.93
2002	5.01
2003	5.1
2004	5.19
2005	5.28
2006	5.37
2007	5.47
2008	5.56
2009	5.66
2010	5.76
2011	5.87
2012	5.97
2013	6.08
2014	6.19
2015	6.3