Year	Years
1970	1.94
1971	2
1972	2.07
1973	2.14
1974	2.22
1975	2.29
1976	2.37
1977	2.46
1978	2.54
1979	2.63
1980	2.72
1981	2.81
1982	2.91
1983	3.01
1984	3.12
1985	3.22
1986	3.34
1987	3.45
1988	3.57
1989	3.7
1990	3.83
1991	3.96
1992	4.1
1993	4.24
1994	4.38
1995	4.53
1996	4.69
1997	4.84
1998	5
1999	5.15
2000	5.31
2001	5.47
2002	5.63
2003	5.79
2004	5.95
2005	6.11
2006	6.27
2007	6.43
2008	6.59
2009	6.76
2010	6.92
2011	7.08
2012	7.24
2013	7.4
2014	7.57
2015	7.73