Year	Years
1970	3.06
1971	3.14
1972	3.21
1973	3.29
1974	3.37
1975	3.45
1976	3.54
1977	3.62
1978	3.71
1979	3.79
1980	3.88
1981	3.97
1982	4.06
1983	4.15
1984	4.25
1985	4.34
1986	4.43
1987	4.53
1988	4.62
1989	4.72
1990	4.82
1991	4.91
1992	5.01
1993	5.11
1994	5.21
1995	5.31
1996	5.4
1997	5.5
1998	5.6
1999	5.7
2000	5.81
2001	5.91
2002	6.01
2003	6.12
2004	6.23
2005	6.34
2006	6.45
2007	6.56
2008	6.67
2009	6.79
2010	6.9
2011	7.02
2012	7.14
2013	7.26
2014	7.38
2015	7.5