year	count
1952	5.4
1953	4
1954	4.8
1955	7.1
1956	5.2
1957	3.8
1958	3.1
1959	3.5
1960	3.4
1961	3.2
1962	3.1
1963	2.1
1964	1.6
1965	1.25
1966	1.6
1967	1.1
1968	2.9
1969	1.6
1970	1.2
1971	1.3
1972	1
1973	0.8
1974	0.6
1975	0.6
1976	0.6
1977	0.9
1978	0.6
1979	0.4
1980	0.4
1981	0.3
1982	0.5
1983	0.5
1984	0.5
1985	0.6
1986	0.7
1987	0.6
1988	0.8
1989	0.4
1990	0.6
1991	0.6
1992	0.7
1993	0.6
1994	0.6
1995	0.55
1996	0.65
1997	0.6
1998	0.65
1999	0.6
2000	0.6