Year	Years
1970	3.9
1971	4
1972	4.11
1973	4.22
1974	4.33
1975	4.44
1976	4.56
1977	4.67
1978	4.79
1979	4.91
1980	5.03
1981	5.15
1982	5.28
1983	5.4
1984	5.53
1985	5.66
1986	5.79
1987	5.92
1988	6.05
1989	6.19
1990	6.32
1991	6.46
1992	6.6
1993	6.74
1994	6.88
1995	7.02
1996	7.17
1997	7.31
1998	7.46
1999	7.61
2000	7.77
2001	7.92
2002	8.08
2003	8.24
2004	8.4
2005	8.56
2006	8.72
2007	8.89
2008	9.05
2009	9.22
2010	9.39
2011	9.56
2012	9.73
2013	9.9
2014	10.07
2015	10.24