Year	Years
1970	4.32
1971	4.39
1972	4.46
1973	4.53
1974	4.6
1975	4.67
1976	4.74
1977	4.81
1978	4.89
1979	4.96
1980	5.03
1981	5.1
1982	5.17
1983	5.25
1984	5.32
1985	5.4
1986	5.48
1987	5.56
1988	5.64
1989	5.73
1990	5.82
1991	5.92
1992	6.02
1993	6.13
1994	6.25
1995	6.38
1996	6.52
1997	6.66
1998	6.8
1999	6.95
2000	7.09
2001	7.23
2002	7.38
2003	7.52
2004	7.66
2005	7.81
2006	7.95
2007	8.09
2008	8.24
2009	8.38
2010	8.52
2011	8.67
2012	8.81
2013	8.96
2014	9.1
2015	9.24