Year	Years
1970	2.26
1971	2.32
1972	2.37
1973	2.43
1974	2.48
1975	2.54
1976	2.59
1977	2.65
1978	2.71
1979	2.77
1980	2.83
1981	2.88
1982	2.94
1983	3
1984	3.06
1985	3.12
1986	3.18
1987	3.23
1988	3.29
1989	3.35
1990	3.4
1991	3.46
1992	3.51
1993	3.56
1994	3.61
1995	3.66
1996	3.71
1997	3.77
1998	3.82
1999	3.88
2000	3.94
2001	4
2002	4.07
2003	4.14
2004	4.21
2005	4.28
2006	4.36
2007	4.44
2008	4.52
2009	4.6
2010	4.69
2011	4.77
2012	4.86
2013	4.95
2014	5.05
2015	5.15