Year	Percent
1970	28.63
1971	24.86
1972	25.91
1973	30.44
1974	28.14
1975	28.96
1976	38.07
1977	37.07
1978	38.16
1979	35.94
1983	40.11
1984	41.59
1989	46.23
1991	46.8
1993	42.28
1994	45.33
1995	41.11
1997	49.07
1999	43.87
2000	41.01
2001	44.71
2002	42.81
2003	47.42
2006	52.12
2007	51.8
2011	55.33