Year	Years
1970	3.61
1971	3.71
1972	3.82
1973	3.93
1974	4.04
1975	4.15
1976	4.27
1977	4.39
1978	4.51
1979	4.64
1980	4.76
1981	4.89
1982	5.02
1983	5.16
1984	5.3
1985	5.44
1986	5.58
1987	5.73
1988	5.87
1989	6.02
1990	6.18
1991	6.33
1992	6.49
1993	6.65
1994	6.81
1995	6.98
1996	7.14
1997	7.3
1998	7.46
1999	7.62
2000	7.78
2001	7.95
2002	8.11
2003	8.28
2004	8.44
2005	8.61
2006	8.78
2007	8.94
2008	9.11
2009	9.28
2010	9.45
2011	9.62
2012	9.78
2013	9.95
2014	10.12
2015	10.29