Year	Years
1970	4.14
1971	4.23
1972	4.33
1973	4.42
1974	4.52
1975	4.62
1976	4.72
1977	4.83
1978	4.93
1979	5.03
1980	5.14
1981	5.25
1982	5.36
1983	5.47
1984	5.58
1985	5.7
1986	5.81
1987	5.93
1988	6.05
1989	6.17
1990	6.29
1991	6.41
1992	6.54
1993	6.66
1994	6.79
1995	6.91
1996	7.04
1997	7.17
1998	7.3
1999	7.43
2000	7.56
2001	7.69
2002	7.83
2003	7.96
2004	8.09
2005	8.23
2006	8.37
2007	8.5
2008	8.64
2009	8.78
2010	8.92
2011	9.06
2012	9.19
2013	9.33
2014	9.47
2015	9.61