Year	Years
1970	2.22
1971	2.29
1972	2.35
1973	2.43
1974	2.5
1975	2.57
1976	2.65
1977	2.73
1978	2.81
1979	2.9
1980	2.98
1981	3.07
1982	3.16
1983	3.25
1984	3.34
1985	3.44
1986	3.54
1987	3.64
1988	3.74
1989	3.84
1990	3.95
1991	4.06
1992	4.17
1993	4.28
1994	4.4
1995	4.51
1996	4.63
1997	4.75
1998	4.88
1999	5
2000	5.13
2001	5.26
2002	5.39
2003	5.53
2004	5.66
2005	5.8
2006	5.94
2007	6.08
2008	6.23
2009	6.37
2010	6.52
2011	6.67
2012	6.82
2013	6.97
2014	7.13
2015	7.28