Year	Years
1970	2.93
1971	3.01
1972	3.1
1973	3.2
1974	3.29
1975	3.39
1976	3.49
1977	3.59
1978	3.69
1979	3.8
1980	3.91
1981	4.02
1982	4.13
1983	4.25
1984	4.37
1985	4.49
1986	4.61
1987	4.74
1988	4.86
1989	4.99
1990	5.13
1991	5.26
1992	5.4
1993	5.53
1994	5.67
1995	5.81
1996	5.95
1997	6.09
1998	6.23
1999	6.38
2000	6.52
2001	6.66
2002	6.81
2003	6.96
2004	7.1
2005	7.25
2006	7.4
2007	7.55
2008	7.71
2009	7.86
2010	8.02
2011	8.17
2012	8.33
2013	8.49
2014	8.65
2015	8.81