Year	Number
1971	15.28205
1972	11.06716
1973	14.46897
1974	15.48193
1975	13.36444
1976	19.69091
1978	17.98031
1979	20.74144
1980	22.6755
1981	24.84589
1982	25.10169
1983	21.86343
1984	17.6708
1985	19.1633
1986	17.40065
1987	16.14076
1988	15.38059
1989	16.08424
1990	11.92045
1991	9.59622
1992	10.39258
1993	10.45117
1999	24.23756
2000	24.23101
2001	16.38509
2002	13.93356
2003	13.81096
2009	19.99744
2011	11.52837
2012	11.84896
2013	12.25985
2017	12.20142