Year	Years
1970	4.18
1971	4.28
1972	4.37
1973	4.47
1974	4.57
1975	4.67
1976	4.77
1977	4.87
1978	4.98
1979	5.08
1980	5.19
1981	5.3
1982	5.41
1983	5.52
1984	5.63
1985	5.75
1986	5.86
1987	5.98
1988	6.09
1989	6.21
1990	6.33
1991	6.45
1992	6.58
1993	6.7
1994	6.82
1995	6.94
1996	7.06
1997	7.19
1998	7.31
1999	7.43
2000	7.56
2001	7.69
2002	7.81
2003	7.94
2004	8.07
2005	8.2
2006	8.33
2007	8.47
2008	8.6
2009	8.73
2010	8.87
2011	9
2012	9.14
2013	9.28
2014	9.41
2015	9.55