Year	Years
1970	3.66
1971	3.77
1972	3.88
1973	3.99
1974	4.1
1975	4.22
1976	4.34
1977	4.46
1978	4.58
1979	4.71
1980	4.84
1981	4.97
1982	5.11
1983	5.24
1984	5.38
1985	5.53
1986	5.67
1987	5.82
1988	5.97
1989	6.13
1990	6.28
1991	6.44
1992	6.61
1993	6.77
1994	6.94
1995	7.11
1996	7.27
1997	7.44
1998	7.6
1999	7.76
2000	7.93
2001	8.09
2002	8.26
2003	8.43
2004	8.59
2005	8.76
2006	8.93
2007	9.1
2008	9.27
2009	9.44
2010	9.61
2011	9.77
2012	9.94
2013	10.11
2014	10.28
2015	10.45