Year	Percent
1971	14.46
1972	11.27
1973	9.95
1974	8.523
1975	8.789
1976	8.297
1977	8.099
1978	8.564
1979	8.922
1980	9.041
1981	8.426
1982	8.229
1983	8.386
1984	7.487
1985	6.187
1986	6.847
1987	6.167
1988	6.39
1989	6.251
1990	5.569
1991	6.374
1992	5.821
1993	6.082
1994	6.206
1995	5.278
1996	5.375
1997	4.552
1998	5.32
1999	4.958
2000	5.619
2001	5.438
2002	5.729
2003	4.701
2004	4.089
2005	3.993
2006	3.743
2007	3.671
2008	3.495
2009	3.165
2010	2.851