Year	Years
1970	2.58
1971	2.66
1972	2.74
1973	2.83
1974	2.92
1975	3.01
1976	3.1
1977	3.2
1978	3.3
1979	3.41
1980	3.52
1981	3.63
1982	3.74
1983	3.86
1984	3.98
1985	4.11
1986	4.24
1987	4.38
1988	4.52
1989	4.67
1990	4.82
1991	4.98
1992	5.14
1993	5.31
1994	5.49
1995	5.68
1996	5.85
1997	6.03
1998	6.2
1999	6.38
2000	6.55
2001	6.73
2002	6.9
2003	7.08
2004	7.26
2005	7.43
2006	7.61
2007	7.78
2008	7.96
2009	8.14
2010	8.31
2011	8.49
2012	8.67
2013	8.84
2014	9.02
2015	9.2