Year	Years
1970	4.39
1971	4.49
1972	4.6
1973	4.7
1974	4.8
1975	4.91
1976	5.02
1977	5.13
1978	5.24
1979	5.35
1980	5.47
1981	5.59
1982	5.71
1983	5.83
1984	5.95
1985	6.07
1986	6.2
1987	6.32
1988	6.45
1989	6.58
1990	6.71
1991	6.84
1992	6.98
1993	7.11
1994	7.24
1995	7.38
1996	7.51
1997	7.65
1998	7.78
1999	7.92
2000	8.05
2001	8.19
2002	8.33
2003	8.47
2004	8.6
2005	8.74
2006	8.88
2007	9.02
2008	9.16
2009	9.29
2010	9.43
2011	9.57
2012	9.71
2013	9.85
2014	9.99
2015	10.12