Year	Years
1970	4.23
1971	4.33
1972	4.43
1973	4.53
1974	4.64
1975	4.74
1976	4.85
1977	4.96
1978	5.07
1979	5.18
1980	5.3
1981	5.41
1982	5.53
1983	5.65
1984	5.77
1985	5.89
1986	6.02
1987	6.14
1988	6.27
1989	6.4
1990	6.53
1991	6.66
1992	6.79
1993	6.93
1994	7.06
1995	7.2
1996	7.34
1997	7.47
1998	7.61
1999	7.75
2000	7.88
2001	8.02
2002	8.16
2003	8.3
2004	8.44
2005	8.58
2006	8.71
2007	8.85
2008	8.99
2009	9.13
2010	9.27
2011	9.41
2012	9.55
2013	9.69
2014	9.83
2015	9.96