Year	Years
1970	3.27
1971	3.34
1972	3.41
1973	3.47
1974	3.54
1975	3.61
1976	3.68
1977	3.75
1978	3.82
1979	3.89
1980	3.96
1981	4.04
1982	4.11
1983	4.18
1984	4.26
1985	4.33
1986	4.41
1987	4.49
1988	4.57
1989	4.65
1990	4.74
1991	4.83
1992	4.92
1993	5.02
1994	5.12
1995	5.22
1996	5.34
1997	5.46
1998	5.57
1999	5.69
2000	5.81
2001	5.93
2002	6.06
2003	6.18
2004	6.3
2005	6.43
2006	6.56
2007	6.69
2008	6.82
2009	6.95
2010	7.08
2011	7.21
2012	7.34
2013	7.48
2014	7.61
2015	7.75