Year	Years
1970	4.1
1971	4.2
1972	4.3
1973	4.4
1974	4.5
1975	4.5
1976	4.6
1977	4.7
1978	4.8
1979	4.9
1980	4.9
1981	5
1982	5.1
1983	5.2
1984	5.2
1985	5.3
1986	5.4
1987	5.5
1988	5.5
1989	5.6
1990	5.7
1991	5.7
1992	5.8
1993	5.9
1994	6
1995	6
1996	6.1
1997	6.2
1998	6.3
1999	6.3
2000	6.4
2001	6.5
2002	6.6
2003	6.6
2004	6.7
2005	6.8
2006	6.8
2007	6.9
2008	7
2009	7.1