Year	Number
1971	9.03125
1972	9.45455
1973	9.51351
1974	14.23256
1975	9.98507
1976	8.89873
1977	12.13861
1980	10.512
1981	12
1982	10.2087
1985	9.08798
1986	9.63134
1987	6.57396
1988	9.42967
1991	12.33034
1992	11.81322
1993	13.33849
1994	11.71908
2003	22.26254
2012	22.09384
2013	19.74135
2014	17.48142
2015	15.4369
2016	18.9776
2017	19.39918
2018	21.85798