Year	Number
1970	26.26
1971	25.02538
1972	23.95313
1973	25.98837
1974	26.02703
1975	25.7905
1976	28.4539
1977	26.03289
1978	25.78366
1979	26.51304
1980	25.83664
1981	21.19275
1982	19.55249
1990	17.97573
1993	19.18838
1996	13.95059
1997	16.57805
1998	16.5735
1999	16.37009
2000	16.02614
2001	15.70668
2002	15.27551
2003	15.39872
2004	15.15077
2005	14.93643
2006	14.69292
2007	14.51134
2008	14.26289
2009	14.28674
2010	14.4669
2011	13.95725
2012	13.19087
2013	13.73208
2014	13.39049
2015	14.0188
2016	12.79521
2017	12.29202
2018	12.37967