Year	Years
1970	2.33
1971	2.39
1972	2.45
1973	2.51
1974	2.58
1975	2.64
1976	2.71
1977	2.77
1978	2.84
1979	2.91
1980	2.99
1981	3.06
1982	3.13
1983	3.21
1984	3.29
1985	3.37
1986	3.45
1987	3.53
1988	3.62
1989	3.71
1990	3.79
1991	3.88
1992	3.98
1993	4.07
1994	4.16
1995	4.26
1996	4.36
1997	4.47
1998	4.57
1999	4.67
2000	4.78
2001	4.89
2002	4.99
2003	5.1
2004	5.21
2005	5.33
2006	5.44
2007	5.55
2008	5.67
2009	5.79
2010	5.9
2011	6.02
2012	6.14
2013	6.27
2014	6.39
2015	6.51