Year	Years
1970	3.7
1971	3.79
1972	3.88
1973	3.97
1974	4.07
1975	4.17
1976	4.27
1977	4.37
1978	4.47
1979	4.57
1980	4.68
1981	4.79
1982	4.9
1983	5.01
1984	5.13
1985	5.24
1986	5.36
1987	5.48
1988	5.6
1989	5.72
1990	5.85
1991	5.97
1992	6.1
1993	6.23
1994	6.36
1995	6.49
1996	6.62
1997	6.75
1998	6.88
1999	7.01
2000	7.14
2001	7.28
2002	7.41
2003	7.54
2004	7.67
2005	7.81
2006	7.94
2007	8.07
2008	8.21
2009	8.34
2010	8.47
2011	8.61
2012	8.74
2013	8.87
2014	9
2015	9.14