Year	Years
1970	2.54
1971	2.63
1972	2.71
1973	2.8
1974	2.9
1975	2.99
1976	3.1
1977	3.2
1978	3.31
1979	3.42
1980	3.53
1981	3.65
1982	3.77
1983	3.9
1984	4.03
1985	4.17
1986	4.31
1987	4.45
1988	4.6
1989	4.76
1990	4.92
1991	5.09
1992	5.26
1993	5.44
1994	5.62
1995	5.81
1996	5.97
1997	6.13
1998	6.3
1999	6.46
2000	6.62
2001	6.79
2002	6.95
2003	7.12
2004	7.29
2005	7.45
2006	7.62
2007	7.79
2008	7.96
2009	8.12
2010	8.29
2011	8.46
2012	8.63
2013	8.8
2014	8.97
2015	9.14