Year	Years
1970	3.67
1971	3.76
1972	3.84
1973	3.93
1974	4.02
1975	4.12
1976	4.21
1977	4.31
1978	4.4
1979	4.5
1980	4.6
1981	4.71
1982	4.81
1983	4.91
1984	5.02
1985	5.13
1986	5.24
1987	5.35
1988	5.47
1989	5.58
1990	5.7
1991	5.81
1992	5.93
1993	6.05
1994	6.17
1995	6.3
1996	6.42
1997	6.54
1998	6.66
1999	6.78
2000	6.91
2001	7.03
2002	7.16
2003	7.29
2004	7.42
2005	7.54
2006	7.68
2007	7.81
2008	7.94
2009	8.07
2010	8.21
2011	8.34
2012	8.48
2013	8.61
2014	8.75
2015	8.89