Year	Years
1970	4.7
1971	4.8
1972	5
1973	5.1
1974	5.2
1975	5.3
1976	5.5
1977	5.6
1978	5.7
1979	5.8
1980	5.9
1981	6
1982	6.2
1983	6.3
1984	6.4
1985	6.5
1986	6.6
1987	6.7
1988	6.8
1989	6.9
1990	7
1991	7.1
1992	7.2
1993	7.3
1994	7.4
1995	7.5
1996	7.6
1997	7.7
1998	7.8
1999	7.9
2000	8
2001	8.1
2002	8.2
2003	8.3
2004	8.4
2005	8.5
2006	8.6
2007	8.7
2008	8.8
2009	8.9