Year	Number
1971	20.71064
1972	20.21637
1973	19.74015
1974	19.36652
1976	18.44955
1977	18.52453
1978	18.49932
1979	19.44978
1981	16.51865
1982	20.74489
1984	20.20301
1987	19.63449
1988	19.49293
1989	19.36195
1990	17.96875
1991	16.67231
1992	16.20211
1994	17.05838
1996	19.18318
1997	18.31527
1998	19.42817
1999	18.26947
2000	18.44818
2001	17.45542
2002	17.40515
2003	17.775
2004	16.37427
2005	16.27439
2006	15.88598
2007	15.68405
2008	15.3172
2009	14.60815
2010	14.4885
2011	14.55048
2012	14.58702
2014	14.36696
2015	14.49628
2016	14.6225
2017	14.91552