Year	Years
1970	3.11
1971	3.2
1972	3.29
1973	3.38
1974	3.48
1975	3.57
1976	3.67
1977	3.77
1978	3.88
1979	3.98
1980	4.09
1981	4.2
1982	4.31
1983	4.42
1984	4.53
1985	4.65
1986	4.77
1987	4.89
1988	5
1989	5.13
1990	5.25
1991	5.37
1992	5.49
1993	5.62
1994	5.74
1995	5.87
1996	5.99
1997	6.11
1998	6.24
1999	6.36
2000	6.49
2001	6.63
2002	6.76
2003	6.9
2004	7.03
2005	7.17
2006	7.32
2007	7.46
2008	7.61
2009	7.75
2010	7.9
2011	8.05
2012	8.2
2013	8.35
2014	8.51
2015	8.66