Year	Percent
1957	4
1958	4
1959	4
1960	4
1961	4
1962	4
1963	4
1964	4
1965	4
1966	4
1967	4
1968	4
1969	4
1970	4
1971	4
1972	4
1973	4
1974	5
1975	5
1976	5
1977	7
1978	5
1979	9
1980	8
1981	12
1982	9
1983	9
1984	9
1985	9
1986	9
1987	9
1988	9
1989	13
1990	19
1991	17