Year	Years
1970	4.61
1971	4.71
1972	4.82
1973	4.92
1974	5.03
1975	5.14
1976	5.25
1977	5.36
1978	5.47
1979	5.59
1980	5.7
1981	5.82
1982	5.94
1983	6.06
1984	6.18
1985	6.31
1986	6.43
1987	6.56
1988	6.69
1989	6.82
1990	6.95
1991	7.08
1992	7.21
1993	7.34
1994	7.48
1995	7.61
1996	7.74
1997	7.88
1998	8.01
1999	8.14
2000	8.28
2001	8.42
2002	8.55
2003	8.69
2004	8.83
2005	8.96
2006	9.1
2007	9.24
2008	9.38
2009	9.52
2010	9.66
2011	9.79
2012	9.93
2013	10.07
2014	10.21
2015	10.35