Year	Number
1971	24.76156
1972	24.62055
1973	25.28533
1974	24.64307
1975	23.21858
1976	23.55859
1977	21.26048
1978	19.96269
1979	18.61193
1980	19.28798
1981	19.84902
1982	20.33502
1983	19.0717
1984	19.51631
1985	18.39712
1986	19.98892
1987	26.96505
1988	25.56028
1990	18.04747
1991	18.70619
1994	18.95925
2000	16.49342
2001	16.98883
2002	16.87323
2003	17.43373
2004	17.52833
2005	15.40671
2006	14.99725
2007	15.89776
2008	15.66096
2009	15.73709
2010	15.65153
2011	15.77947
2012	16.13468
2013	16.32882
2014	17.05417
2015	17.20247
2016	16.9637
2017	16.56074