Year	Number
1971	12.18364
1972	9.71385
1973	10.11329
1974	10.23521
1975	10.87624
1976	12.39428
1977	11.03473
1978	11.07392
1979	10.09444
1980	9.72998
1981	8.33432
1982	8.78491
1983	9.18394
1984	9.04159
1985	9.79671
1986	10.39357
1987	9.9374
1989	11.02734
1990	11.90513
1991	11.61139
1992	14.01198
1993	13.01257
1994	13.9704
1996	16.81202
1997	17.26695
1998	17.81419
1999	18.8609
2000	19.91297
2001	20.90557
2002	20.29382
2003	22.49
2004	22.91454
2005	22.49855
2006	22.75944
2007	20.70845
2008	19.95525
2009	19.22981
2010	18.24356
2011	18.85443
2012	20.2888
2013	20.97114
2014	20.27622
2015	19.84502
2016	20.33542
2017	20.03854
2018	18.9733