Year	Years
1970	3.68
1971	3.77
1972	3.86
1973	3.95
1974	4.04
1975	4.13
1976	4.23
1977	4.32
1978	4.42
1979	4.52
1980	4.62
1981	4.73
1982	4.83
1983	4.94
1984	5.05
1985	5.16
1986	5.27
1987	5.39
1988	5.5
1989	5.62
1990	5.74
1991	5.86
1992	5.98
1993	6.11
1994	6.24
1995	6.36
1996	6.49
1997	6.62
1998	6.74
1999	6.87
2000	7
2001	7.13
2002	7.26
2003	7.39
2004	7.52
2005	7.66
2006	7.79
2007	7.92
2008	8.06
2009	8.19
2010	8.33
2011	8.47
2012	8.61
2013	8.74
2014	8.88
2015	9.02