Year	Years
1970	3.54
1971	3.64
1972	3.73
1973	3.83
1974	3.92
1975	4.02
1976	4.11
1977	4.21
1978	4.31
1979	4.41
1980	4.5
1981	4.6
1982	4.7
1983	4.8
1984	4.89
1985	4.99
1986	5.08
1987	5.17
1988	5.26
1989	5.34
1990	5.42
1991	5.5
1992	5.57
1993	5.64
1994	5.7
1995	5.76
1996	5.85
1997	5.96
1998	6.07
1999	6.18
2000	6.29
2001	6.41
2002	6.53
2003	6.66
2004	6.78
2005	6.92
2006	7.05
2007	7.19
2008	7.33
2009	7.47
2010	7.61
2011	7.76
2012	7.91
2013	8.06
2014	8.21
2015	8.37