Year	Years
1970	2.03
1971	2.07
1972	2.11
1973	2.16
1974	2.2
1975	2.24
1976	2.29
1977	2.33
1978	2.38
1979	2.42
1980	2.47
1981	2.51
1982	2.55
1983	2.6
1984	2.64
1985	2.69
1986	2.73
1987	2.77
1988	2.82
1989	2.86
1990	2.9
1991	2.94
1992	2.99
1993	3.03
1994	3.07
1995	3.12
1996	3.18
1997	3.25
1998	3.32
1999	3.38
2000	3.46
2001	3.53
2002	3.61
2003	3.68
2004	3.76
2005	3.84
2006	3.93
2007	4.01
2008	4.1
2009	4.18
2010	4.27
2011	4.37
2012	4.46
2013	4.55
2014	4.65
2015	4.75