Year	Years
1970	2.5
1971	2.59
1972	2.68
1973	2.77
1974	2.86
1975	2.96
1976	3.06
1977	3.17
1978	3.27
1979	3.39
1980	3.5
1981	3.62
1982	3.74
1983	3.86
1984	3.99
1985	4.13
1986	4.26
1987	4.4
1988	4.54
1989	4.69
1990	4.84
1991	5
1992	5.15
1993	5.31
1994	5.47
1995	5.63
1996	5.78
1997	5.92
1998	6.07
1999	6.22
2000	6.37
2001	6.52
2002	6.67
2003	6.82
2004	6.98
2005	7.13
2006	7.29
2007	7.45
2008	7.61
2009	7.77
2010	7.93
2011	8.09
2012	8.26
2013	8.42
2014	8.58
2015	8.75