Year	Number
1971	25.64444
1972	28.91002
1973	28.84724
1974	30.59447
1975	29.90951
1976	30.88596
1977	31.29589
1978	28.32871
1979	26.64064
1980	25.69824
1986	18.87847
1987	19.02698
1989	20.3188
1996	20.25164
1998	19.9444
1999	19.56571
2000	19.39514
2001	19.23417
2002	18.83802
2003	18.55815
2004	17.57327
2005	17.41396
2006	16.61191
2007	16.05896
2008	16.05079
2009	15.71952
2010	15.25916
2011	14.9129
2012	14.7082
2013	13.12128
2014	13.26561
2015	12.87505
2017	12.65226
2018	10.9886