Year	Years
1970	3.98
1971	4.07
1972	4.16
1973	4.26
1974	4.35
1975	4.45
1976	4.55
1977	4.65
1978	4.75
1979	4.86
1980	4.96
1981	5.07
1982	5.18
1983	5.29
1984	5.4
1985	5.52
1986	5.63
1987	5.75
1988	5.87
1989	5.99
1990	6.12
1991	6.24
1992	6.37
1993	6.49
1994	6.62
1995	6.76
1996	6.9
1997	7.04
1998	7.18
1999	7.32
2000	7.46
2001	7.6
2002	7.74
2003	7.88
2004	8.02
2005	8.17
2006	8.31
2007	8.45
2008	8.6
2009	8.74
2010	8.88
2011	9.03
2012	9.17
2013	9.31
2014	9.46
2015	9.6