Year	Years
1970	3.41
1971	3.5
1972	3.58
1973	3.66
1974	3.75
1975	3.84
1976	3.93
1977	4.02
1978	4.11
1979	4.2
1980	4.3
1981	4.4
1982	4.5
1983	4.6
1984	4.7
1985	4.8
1986	4.91
1987	5.02
1988	5.12
1989	5.23
1990	5.35
1991	5.46
1992	5.57
1993	5.69
1994	5.81
1995	5.93
1996	6.05
1997	6.17
1998	6.29
1999	6.41
2000	6.54
2001	6.66
2002	6.79
2003	6.92
2004	7.04
2005	7.17
2006	7.3
2007	7.44
2008	7.57
2009	7.7
2010	7.83
2011	7.97
2012	8.1
2013	8.24
2014	8.38
2015	8.51