Year	Years
1970	2.53
1971	2.6
1972	2.67
1973	2.75
1974	2.83
1975	2.9
1976	2.99
1977	3.07
1978	3.16
1979	3.24
1980	3.34
1981	3.43
1982	3.53
1983	3.63
1984	3.73
1985	3.84
1986	3.95
1987	4.06
1988	4.17
1989	4.29
1990	4.42
1991	4.54
1992	4.67
1993	4.81
1994	4.94
1995	5.08
1996	5.22
1997	5.36
1998	5.5
1999	5.64
2000	5.78
2001	5.92
2002	6.06
2003	6.2
2004	6.34
2005	6.48
2006	6.62
2007	6.76
2008	6.9
2009	7.04
2010	7.18
2011	7.32
2012	7.46
2013	7.6
2014	7.73
2015	7.87