Year	Years
1970	2.6
1971	2.68
1972	2.76
1973	2.85
1974	2.94
1975	3.04
1976	3.13
1977	3.24
1978	3.34
1979	3.45
1980	3.57
1981	3.68
1982	3.81
1983	3.93
1984	4.07
1985	4.2
1986	4.35
1987	4.5
1988	4.65
1989	4.81
1990	4.98
1991	5.16
1992	5.34
1993	5.53
1994	5.73
1995	5.93
1996	6.07
1997	6.21
1998	6.35
1999	6.49
2000	6.63
2001	6.76
2002	6.89
2003	7.03
2004	7.16
2005	7.29
2006	7.42
2007	7.55
2008	7.68
2009	7.81
2010	7.94
2011	8.07
2012	8.2
2013	8.33
2014	8.46
2015	8.6