Year	Years
1970	3.8
1971	3.9
1972	4
1973	4.1
1974	4.2
1975	4.3
1976	4.4
1977	4.4
1978	4.5
1979	4.6
1980	4.7
1981	4.8
1982	4.9
1983	5
1984	5.1
1985	5.1
1986	5.2
1987	5.3
1988	5.4
1989	5.4
1990	5.5
1991	5.6
1992	5.6
1993	5.7
1994	5.8
1995	5.9
1996	5.9
1997	6
1998	6.1
1999	6.2
2000	6.2
2001	6.3
2002	6.4
2003	6.5
2004	6.5
2005	6.6
2006	6.7
2007	6.8
2008	6.8
2009	6.9