Year	Years
1970	3.5
1971	3.6
1972	3.7
1973	3.8
1974	3.9
1975	4
1976	4.1
1977	4.2
1978	4.2
1979	4.3
1980	4.4
1981	4.5
1982	4.6
1983	4.6
1984	4.7
1985	4.8
1986	4.8
1987	4.7
1988	4.7
1989	4.7
1990	4.7
1991	4.7
1992	4.8
1993	4.8
1994	4.9
1995	4.9
1996	5
1997	5
1998	5.1
1999	5.2
2000	5.3
2001	5.4
2002	5.5
2003	5.5
2004	5.6
2005	5.8
2006	5.9
2007	6
2008	6.1
2009	6.2