Year	Years
1970	2.76
1971	2.84
1972	2.93
1973	3.02
1974	3.11
1975	3.21
1976	3.3
1977	3.4
1978	3.51
1979	3.61
1980	3.72
1981	3.83
1982	3.94
1983	4.06
1984	4.17
1985	4.29
1986	4.42
1987	4.54
1988	4.67
1989	4.8
1990	4.93
1991	5.07
1992	5.2
1993	5.34
1994	5.48
1995	5.63
1996	5.77
1997	5.92
1998	6.06
1999	6.21
2000	6.36
2001	6.51
2002	6.66
2003	6.81
2004	6.96
2005	7.12
2006	7.27
2007	7.43
2008	7.58
2009	7.74
2010	7.9
2011	8.05
2012	8.21
2013	8.37
2014	8.53
2015	8.69