Year	Years
1970	2.54
1971	2.62
1972	2.7
1973	2.78
1974	2.86
1975	2.95
1976	3.04
1977	3.13
1978	3.22
1979	3.32
1980	3.41
1981	3.51
1982	3.62
1983	3.72
1984	3.83
1985	3.94
1986	4.06
1987	4.17
1988	4.29
1989	4.42
1990	4.54
1991	4.67
1992	4.8
1993	4.94
1994	5.07
1995	5.22
1996	5.36
1997	5.5
1998	5.65
1999	5.79
2000	5.94
2001	6.09
2002	6.24
2003	6.39
2004	6.55
2005	6.7
2006	6.86
2007	7.02
2008	7.18
2009	7.34
2010	7.5
2011	7.66
2012	7.82
2013	7.99
2014	8.15
2015	8.32