Year	Number
1971	21.43599
1972	21.53882
1973	21.26926
1974	20.63155
1975	20.27134
1976	19.02794
1978	16.38821
1979	17.08714
1980	15.96325
1981	14.54667
1982	13.96851
1983	13.2592
1984	12.68624
1985	12.16607
1986	10.48049
1987	10.36293
1988	10.52307
1989	10.73233
1990	10.85945
1991	10.81473
1992	10.84969
1993	11.42039
1994	11.90393
1995	11.68606
1996	12.45371
1997	12.22127
1998	12.14873
1999	13.45715
2002	13.26801
2003	13.1816
2004	13.13389
2005	12.40884
2006	12.23292
2007	11.86203
2008	11.62932
2009	11.36314
2010	11.02831
2011	10.89398
2012	10.82489
2013	10.7043
2014	10.71766
2015	10.58415
2016	10.30374
2017	10.01653