Year	Years
1970	4.61
1971	4.71
1972	4.81
1973	4.92
1974	5.02
1975	5.13
1976	5.24
1977	5.34
1978	5.45
1979	5.57
1980	5.68
1981	5.79
1982	5.91
1983	6.03
1984	6.15
1985	6.27
1986	6.39
1987	6.51
1988	6.64
1989	6.76
1990	6.89
1991	7.01
1992	7.14
1993	7.27
1994	7.4
1995	7.53
1996	7.66
1997	7.8
1998	7.93
1999	8.06
2000	8.2
2001	8.33
2002	8.47
2003	8.61
2004	8.75
2005	8.88
2006	9.02
2007	9.16
2008	9.3
2009	9.44
2010	9.58
2011	9.72
2012	9.86
2013	10
2014	10.14
2015	10.28