Year	Number
1972	34
1973	28.4
1978	27.74118
1979	19.64943
1980	18.70335
1981	21.01587
1982	17.89706
1983	14.38313
1984	16.73266
1985	20.17125
1986	16.29616
1988	19.98389
1990	18.54312
1991	19.5932
1992	15.86836
1993	15.60627
1994	15.7743
1995	16.04711
1996	17.4584
1997	17.41878
1998	17.79352
1999	17.59381
2000	17.13651
2001	15.30633
2002	15.08657
2003	15.32941
2004	15.3921
2005	15.28618
2006	15.37792
2007	15.47489
2008	17.64946
2009	23.63408
2010	19.01398
2011	16.41408
2012	18.88959
2014	19.18014
2015	18.71411
2016	18.27826
2017	18.24073
2018	18.24345