Year	Years
1970	4.26
1971	4.35
1972	4.46
1973	4.56
1974	4.66
1975	4.77
1976	4.88
1977	4.99
1978	5.1
1979	5.22
1980	5.33
1981	5.45
1982	5.57
1983	5.69
1984	5.82
1985	5.94
1986	6.07
1987	6.2
1988	6.33
1989	6.47
1990	6.6
1991	6.74
1992	6.87
1993	7.01
1994	7.15
1995	7.3
1996	7.43
1997	7.57
1998	7.71
1999	7.84
2000	7.98
2001	8.12
2002	8.26
2003	8.4
2004	8.54
2005	8.67
2006	8.81
2007	8.95
2008	9.09
2009	9.23
2010	9.37
2011	9.51
2012	9.66
2013	9.8
2014	9.94
2015	10.08