Year	Number
1971	38.23014
1972	35.37616
1974	46.27715
1975	42.31481
1976	52.12711
1977	45.95586
1978	42.13739
1979	37.90271
1980	41.62679
1981	40.10237
1984	38.58201
1986	38.78701
1987	33.62702
1988	33.15381
1989	33.00202
1990	33.41272
1991	36.83175
1994	29.42153
1998	28.51103
1999	28.66264
2000	28.72602
2001	28.20162
2002	28.77571
2003	27.931
2004	26.86589
2005	26.01411
2006	25.36885
2007	24.85836
2008	24.36111
2009	23.94141
2010	23.64041
2011	23.25538
2012	22.97124
2013	22.92555
2014	22.60472
2015	21.94779
2016	21.4778
2017	21.08156
2018	21.07361