Year	Years
1970	4.51
1971	4.62
1972	4.72
1973	4.82
1974	4.92
1975	5.02
1976	5.12
1977	5.22
1978	5.32
1979	5.42
1980	5.52
1981	5.61
1982	5.71
1983	5.8
1984	5.89
1985	5.97
1986	6.05
1987	6.12
1988	6.19
1989	6.26
1990	6.32
1991	6.37
1992	6.41
1993	6.45
1994	6.48
1995	6.51
1996	6.62
1997	6.73
1998	6.84
1999	6.96
2000	7.09
2001	7.22
2002	7.35
2003	7.49
2004	7.62
2005	7.77
2006	7.91
2007	8.06
2008	8.21
2009	8.36
2010	8.51
2011	8.66
2012	8.82
2013	8.98
2014	9.14
2015	9.3