Year	Years
1970	2.3
1971	2.4
1972	2.4
1973	2.5
1974	2.6
1975	2.7
1976	2.7
1977	2.8
1978	2.9
1979	3
1980	3
1981	3.1
1982	3.2
1983	3.3
1984	3.3
1985	3.4
1986	3.5
1987	3.6
1988	3.6
1989	3.7
1990	3.8
1991	3.8
1992	3.9
1993	4
1994	4
1995	4.1
1996	4.2
1997	4.2
1998	4.3
1999	4.4
2000	4.5
2001	4.5
2002	4.6
2003	4.7
2004	4.7
2005	4.8
2006	4.9
2007	4.9
2008	5
2009	5.1