Year	Number
1971	29.69713
1972	29.20732
1973	29.12487
1974	28.53556
1975	27.58275
1976	27.58002
1977	27.46849
1978	26.70709
1979	25.52793
1980	24.32339
1981	23.17405
1982	22.72735
1983	19.88139
1984	19.89693
1985	19.42453
1986	16.71371
1987	17.02445
1988	17.03894
1989	17.30071
1990	17.36358
1991	17.16896
1992	16.48428
1993	16.16989
1994	18.35756
2011	11.55832
2012	11.50226
2015	11.62356
2016	11.72995
2017	11.80592