Year	Years
1970	3.87
1971	3.96
1972	4.05
1973	4.14
1974	4.23
1975	4.33
1976	4.42
1977	4.52
1978	4.61
1979	4.71
1980	4.81
1981	4.92
1982	5.02
1983	5.12
1984	5.23
1985	5.33
1986	5.44
1987	5.55
1988	5.66
1989	5.77
1990	5.88
1991	5.99
1992	6.11
1993	6.22
1994	6.33
1995	6.45
1996	6.55
1997	6.66
1998	6.76
1999	6.87
2000	6.98
2001	7.1
2002	7.21
2003	7.33
2004	7.44
2005	7.56
2006	7.68
2007	7.8
2008	7.92
2009	8.05
2010	8.17
2011	8.3
2012	8.42
2013	8.55
2014	8.68
2015	8.81