Year	Years
1970	4
1971	4.2
1972	4.4
1973	4.6
1974	4.8
1975	5
1976	5.2
1977	5.4
1978	5.6
1979	5.8
1980	6
1981	6.2
1982	6.4
1983	6.6
1984	6.8
1985	7
1986	7.3
1987	7.5
1988	7.7
1989	7.8
1990	8
1991	8.2
1992	8.4
1993	8.6
1994	8.8
1995	9
1996	9.2
1997	9.4
1998	9.6
1999	9.8
2000	10
2001	10.2
2002	10.4
2003	10.6
2004	10.8
2005	11
2006	11.2
2007	11.4
2008	11.5
2009	11.7