Year	Number
1970	12.39984
1972	10.47002
1973	7.88978
1974	6.04833
1976	6.34966
1977	6.57076
1978	6.57934
1979	6.05643
1980	9.13676
1982	9.33416
1983	8.27336
1984	11.26268
1985	12.96699
1986	13.31427
1987	12.83614
1988	10.27792
1989	11.03733
1990	11.45092
1991	12.36353
1992	13.1417
1993	13.33204
1994	14.09228
1995	14.8053
1996	14.60712
1997	14.9923
1998	13.43293
1999	13.95715
2000	14.35745
2001	13.88153
2002	12.62546
2003	14.74736
2004	13.0776
2005	13.46949
2006	13.68542
2007	14.33613
2008	15.44621
2009	18.4257
2010	18.44949
2011	22.47892
2012	27.53064
2013	27.04396
2014	25.00347
2015	30.54161
2016	29.87432
2017	28.30466
2018	27.26144