Year	Number
1970	29.48589
1971	28.64984
1972	25.66373
1973	25.42803
1974	22.93522
1975	23.76967
1976	23.42196
1977	23.70194
1978	23.57919
1979	23.81625
1980	22.43005
1981	21.48804
1982	21.59081
1983	20.55518
1984	23.32668
1985	25.08293
1986	21.89014
1988	23.17514
1989	22.14033
1990	21.88766
1991	21.64152
1992	20.63866
1993	19.90489
1994	21.39486
1995	20.08104
1996	20.4869
1997	21.59529
1998	20.6103
1999	20.43957
2000	20.75667
2001	20.77524
2002	21.1696
2003	20.82538
2004	20.53409
2005	20.15194
2006	19.64452
2007	15.50977
2008	15.00998
2009	13.98624
2010	13.79403
2013	11.9642
2014	11.46753
2015	11.76053
2016	11.13129
2017	11.0193