1959 19.7 
1967 28 
1970 28.8 
1979 36.4 
1989 38.2 
1992 38 
1996 36.3 
1998 35.4 
2000 34.6 
2001 34.3     
2003 34.2 
2005 33.6       
2006 33.1 
2007 33.0 
2008 32.8 
2009 32.6