Year	Years
1970	3.48
1971	3.57
1972	3.67
1973	3.77
1974	3.87
1975	3.98
1976	4.08
1977	4.19
1978	4.3
1979	4.41
1980	4.52
1981	4.64
1982	4.76
1983	4.88
1984	5
1985	5.12
1986	5.24
1987	5.37
1988	5.5
1989	5.63
1990	5.76
1991	5.89
1992	6.02
1993	6.16
1994	6.29
1995	6.43
1996	6.57
1997	6.71
1998	6.86
1999	7.01
2000	7.15
2001	7.3
2002	7.46
2003	7.61
2004	7.77
2005	7.92
2006	8.08
2007	8.24
2008	8.4
2009	8.57
2010	8.73
2011	8.9
2012	9.06
2013	9.23
2014	9.4
2015	9.57