Year	Years
1970	2.53
1971	2.62
1972	2.71
1973	2.81
1974	2.91
1975	3.01
1976	3.11
1977	3.22
1978	3.33
1979	3.45
1980	3.57
1981	3.69
1982	3.82
1983	3.96
1984	4.09
1985	4.24
1986	4.38
1987	4.53
1988	4.69
1989	4.85
1990	5.02
1991	5.19
1992	5.36
1993	5.53
1994	5.71
1995	5.89
1996	6.06
1997	6.23
1998	6.41
1999	6.58
2000	6.75
2001	6.92
2002	7.09
2003	7.26
2004	7.43
2005	7.61
2006	7.78
2007	7.95
2008	8.12
2009	8.29
2010	8.46
2011	8.63
2012	8.8
2013	8.96
2014	9.13
2015	9.3