Year	Years
1970	4.32
1971	4.42
1972	4.51
1973	4.61
1974	4.71
1975	4.81
1976	4.91
1977	5.01
1978	5.12
1979	5.22
1980	5.33
1981	5.44
1982	5.55
1983	5.66
1984	5.77
1985	5.88
1986	5.99
1987	6.11
1988	6.22
1989	6.34
1990	6.46
1991	6.58
1992	6.7
1993	6.82
1994	6.94
1995	7.06
1996	7.18
1997	7.3
1998	7.43
1999	7.55
2000	7.68
2001	7.81
2002	7.94
2003	8.07
2004	8.2
2005	8.34
2006	8.47
2007	8.61
2008	8.75
2009	8.89
2010	9.03
2011	9.17
2012	9.31
2013	9.45
2014	9.59
2015	9.73