Year	Years
1970	2.28
1971	2.35
1972	2.43
1973	2.51
1974	2.59
1975	2.68
1976	2.76
1977	2.85
1978	2.95
1979	3.04
1980	3.14
1981	3.24
1982	3.34
1983	3.45
1984	3.56
1985	3.67
1986	3.78
1987	3.9
1988	4.02
1989	4.14
1990	4.26
1991	4.39
1992	4.52
1993	4.65
1994	4.78
1995	4.91
1996	5.03
1997	5.16
1998	5.28
1999	5.41
2000	5.55
2001	5.68
2002	5.82
2003	5.96
2004	6.1
2005	6.24
2006	6.39
2007	6.53
2008	6.68
2009	6.83
2010	6.98
2011	7.14
2012	7.29
2013	7.45
2014	7.61
2015	7.77