Year	Years
1970	3.7
1971	3.8
1972	3.8
1973	3.9
1974	4
1975	4.1
1976	4.2
1977	4.3
1978	4.3
1979	4.4
1980	4.5
1981	4.6
1982	4.6
1983	4.7
1984	4.8
1985	4.9
1986	4.9
1987	5
1988	5.1
1989	5.2
1990	5.2
1991	5.3
1992	5.4
1993	5.4
1994	5.5
1995	5.6
1996	5.7
1997	5.7
1998	5.8
1999	5.9
2000	5.9
2001	6
2002	6.1
2003	6.2
2004	6.2
2005	6.3
2006	6.4
2007	6.4
2008	6.5
2009	6.6