Year	Years
1970	2.27
1971	2.34
1972	2.4
1973	2.47
1974	2.54
1975	2.61
1976	2.69
1977	2.76
1978	2.84
1979	2.92
1980	3.01
1981	3.09
1982	3.18
1983	3.27
1984	3.37
1985	3.46
1986	3.56
1987	3.67
1988	3.77
1989	3.88
1990	4
1991	4.12
1992	4.24
1993	4.36
1994	4.49
1995	4.63
1996	4.79
1997	4.94
1998	5.1
1999	5.25
2000	5.4
2001	5.56
2002	5.71
2003	5.86
2004	6.01
2005	6.16
2006	6.31
2007	6.45
2008	6.6
2009	6.75
2010	6.9
2011	7.04
2012	7.19
2013	7.33
2014	7.48
2015	7.62