Year	Number
1970	30.22271
1971	24.80511
1972	23.98705
1973	27.25798
1974	26.57477
1975	27.1291
1976	29.33178
1977	28.64565
1978	24.11982
1979	27.22595
1980	28.11783
1981	26.57579
1982	25.52381
1983	25.43602
1984	24.77175
1985	24.53256
1986	24.22432
1987	23.30558
1988	23.23992
1989	23.40833
1990	21.74027
1991	22.11741
1992	22.89807
1993	23.04534
1994	22.31135
1995	21.99153
1996	22.76957
1999	19.15361
2000	19.24139
2001	18.73312
2002	20.46126
2004	17.55134
2005	19.83345
2006	19.11957
2007	18.37894
2008	17.8604
2009	17.94992
2010	17.32741
2011	18.74088
2012	17.49001
2013	18.62823
2014	20.37883
2015	18.41093
2016	15.0403
2017	15.39271