Year	Percent
1970	21.28
1971	16.52
1972	18.55
1973	16.82
1974	18.93
1975	19.92
1976	28.94
1977	23.8
1978	27.14
1979	31.23
1983	29.24
1984	32.95
1989	27.31
1991	24.99
1993	26.64
1994	31.28
1995	30.51
1997	33.16
1999	28.29
2000	26.09
2001	25.5
2002	26.87
2003	25.6
2006	28.3
2007	29.06
2011	31.43