Year	Number
1971	21.90087
1972	22.36239
1973	22.94899
1974	24.19997
1975	24.72419
1976	24.33527
1977	30.25319
1978	30.15399
1979	30.10794
1980	29.69067
1981	29.33661
1982	28.98703
1983	27.24266
1984	25.93644
1985	24.80259
1986	23.99015
1987	23.37118
1988	22.71436
1996	20.13159
2001	18.83896
2002	18.46881
2003	18.34543
2004	18.03796
2005	18.06874
2006	17.82577
2007	17.4319
2008	17.24002
2009	16.88733
2010	16.67878
2011	15.79139
2012	14.98744
2013	14.19818
2014	13.60538
2015	12.95682
2016	12.37069
2017	11.51862
2018	11.22247