Year	Years
1970	2.51
1971	2.59
1972	2.67
1973	2.75
1974	2.83
1975	2.92
1976	3.01
1977	3.1
1978	3.19
1979	3.28
1980	3.38
1981	3.48
1982	3.59
1983	3.69
1984	3.8
1985	3.91
1986	4.02
1987	4.14
1988	4.26
1989	4.38
1990	4.5
1991	4.63
1992	4.76
1993	4.89
1994	5.02
1995	5.16
1996	5.3
1997	5.44
1998	5.58
1999	5.72
2000	5.87
2001	6.02
2002	6.17
2003	6.31
2004	6.47
2005	6.62
2006	6.77
2007	6.92
2008	7.08
2009	7.23
2010	7.39
2011	7.55
2012	7.7
2013	7.86
2014	8.02
2015	8.18