Year	Number
1971	27.09511
1972	26.37306
1973	22.48931
1974	27.80966
1975	31.09801
1976	29.8489
1977	32.87742
1978	29.92725
1979	28.79476
1980	27.85758
1981	24.98413
1986	25.48225
1988	25.62669
1989	26.77649
1990	25.63037
1991	25.13778
1992	26.41037
1993	27.85714
2001	22.48563
2002	24.78152
2005	23.69596
2006	22.14774
2007	20.03936
2008	21.48462
2010	25.29402
2011	24.07599
2012	28.07395
2013	27.6497
2014	26.4971
2015	26.05739
2016	22.27381
2017	27.73499
2018	19.57929