Year	Years
1970	3.01
1971	3.09
1972	3.17
1973	3.24
1974	3.33
1975	3.41
1976	3.49
1977	3.58
1978	3.67
1979	3.76
1980	3.85
1981	3.94
1982	4.04
1983	4.13
1984	4.23
1985	4.33
1986	4.44
1987	4.54
1988	4.65
1989	4.76
1990	4.87
1991	4.98
1992	5.09
1993	5.21
1994	5.33
1995	5.45
1996	5.57
1997	5.68
1998	5.8
1999	5.92
2000	6.04
2001	6.16
2002	6.28
2003	6.4
2004	6.53
2005	6.65
2006	6.78
2007	6.9
2008	7.03
2009	7.16
2010	7.29
2011	7.42
2012	7.55
2013	7.68
2014	7.81
2015	7.95