Year	Years
1970	2.92
1971	3
1972	3.09
1973	3.18
1974	3.27
1975	3.37
1976	3.47
1977	3.56
1978	3.67
1979	3.77
1980	3.87
1981	3.98
1982	4.09
1983	4.2
1984	4.32
1985	4.44
1986	4.55
1987	4.68
1988	4.8
1989	4.92
1990	5.05
1991	5.18
1992	5.31
1993	5.45
1994	5.58
1995	5.72
1996	5.86
1997	6
1998	6.14
1999	6.29
2000	6.43
2001	6.58
2002	6.73
2003	6.88
2004	7.03
2005	7.19
2006	7.34
2007	7.5
2008	7.66
2009	7.82
2010	7.98
2011	8.14
2012	8.3
2013	8.46
2014	8.63
2015	8.79