Year	Years
1970	2.47
1971	2.53
1972	2.6
1973	2.67
1974	2.75
1975	2.82
1976	2.9
1977	2.98
1978	3.06
1979	3.14
1980	3.23
1981	3.32
1982	3.41
1983	3.51
1984	3.6
1985	3.7
1986	3.81
1987	3.91
1988	4.02
1989	4.13
1990	4.24
1991	4.36
1992	4.48
1993	4.6
1994	4.72
1995	4.85
1996	4.96
1997	5.06
1998	5.17
1999	5.28
2000	5.4
2001	5.51
2002	5.62
2003	5.74
2004	5.85
2005	5.97
2006	6.09
2007	6.21
2008	6.33
2009	6.45
2010	6.58
2011	6.7
2012	6.83
2013	6.95
2014	7.08
2015	7.21