Year	Number
1971	22.05582
1972	22.01632
1973	23.60458
1974	24.1028
1975	26.44791
1976	25.44111
1977	27.59178
1978	26.24348
1979	28.22783
1980	28.176
1981	27.82883
1982	26.79302
1983	24.38507
1984	24.0024
1985	23.82255
1986	23.7781
1987	23.68793
1988	25.12536
1989	23.11669
1991	24.8224
1992	23.52562
1993	21.76769
1996	19.96441
1999	25.47278
2000	21.39027
2003	19.42497
2004	20.52994
2007	16.95545