Year	Years
1970	3.67
1971	3.77
1972	3.88
1973	3.98
1974	4.09
1975	4.21
1976	4.32
1977	4.44
1978	4.55
1979	4.67
1980	4.8
1981	4.92
1982	5.05
1983	5.17
1984	5.3
1985	5.44
1986	5.57
1987	5.7
1988	5.84
1989	5.98
1990	6.12
1991	6.26
1992	6.41
1993	6.55
1994	6.7
1995	6.84
1996	6.99
1997	7.14
1998	7.29
1999	7.45
2000	7.6
2001	7.76
2002	7.91
2003	8.07
2004	8.23
2005	8.39
2006	8.55
2007	8.71
2008	8.88
2009	9.04
2010	9.2
2011	9.37
2012	9.53
2013	9.7
2014	9.86
2015	10.03