Year	Years
1970	2.22
1971	2.29
1972	2.36
1973	2.43
1974	2.51
1975	2.58
1976	2.66
1977	2.74
1978	2.83
1979	2.92
1980	3.01
1981	3.11
1982	3.21
1983	3.32
1984	3.43
1985	3.55
1986	3.67
1987	3.8
1988	3.94
1989	4.08
1990	4.23
1991	4.39
1992	4.56
1993	4.73
1994	4.92
1995	5.11
1996	5.23
1997	5.35
1998	5.47
1999	5.59
2000	5.72
2001	5.84
2002	5.96
2003	6.09
2004	6.22
2005	6.35
2006	6.47
2007	6.6
2008	6.73
2009	6.86
2010	7
2011	7.13
2012	7.26
2013	7.4
2014	7.53
2015	7.67