Year	Years
1970	2.28
1971	2.37
1972	2.47
1973	2.57
1974	2.67
1975	2.78
1976	2.89
1977	3
1978	3.13
1979	3.25
1980	3.38
1981	3.52
1982	3.66
1983	3.81
1984	3.97
1985	4.13
1986	4.29
1987	4.47
1988	4.65
1989	4.83
1990	5.02
1991	5.21
1992	5.41
1993	5.62
1994	5.82
1995	6.03
1996	6.2
1997	6.37
1998	6.53
1999	6.7
2000	6.86
2001	7.02
2002	7.18
2003	7.33
2004	7.49
2005	7.64
2006	7.79
2007	7.94
2008	8.09
2009	8.24
2010	8.39
2011	8.54
2012	8.69
2013	8.84
2014	8.99
2015	9.14