Year	Years
1970	3.34
1971	3.42
1972	3.5
1973	3.58
1974	3.67
1975	3.75
1976	3.84
1977	3.93
1978	4.02
1979	4.11
1980	4.21
1981	4.3
1982	4.4
1983	4.5
1984	4.6
1985	4.71
1986	4.81
1987	4.92
1988	5.03
1989	5.14
1990	5.25
1991	5.37
1992	5.49
1993	5.61
1994	5.73
1995	5.85
1996	5.98
1997	6.11
1998	6.24
1999	6.38
2000	6.51
2001	6.64
2002	6.78
2003	6.92
2004	7.05
2005	7.19
2006	7.33
2007	7.47
2008	7.61
2009	7.75
2010	7.89
2011	8.03
2012	8.17
2013	8.32
2014	8.46
2015	8.6