Year	Years
1970	4.7
1971	4.8
1972	4.9
1973	5
1974	5.1
1975	5.2
1976	5.3
1977	5.4
1978	5.5
1979	5.6
1980	5.7
1981	5.8
1982	5.9
1983	6
1984	6.1
1985	6.2
1986	6.3
1987	6.4
1988	6.5
1989	6.6
1990	6.7
1991	6.7
1992	6.8
1993	6.9
1994	7
1995	7.1
1996	7.1
1997	7.2
1998	7.3
1999	7.4
2000	7.4
2001	7.5
2002	7.6
2003	7.7
2004	7.7
2005	7.8
2006	7.9
2007	7.9
2008	8
2009	8.1