Year	Number
1971	22.70811
1972	23.13636
1973	21.6875
1974	19.81661
1975	21.12393
1978	18.39015
1979	18.49638
1981	17.90458
1982	20.00717
1983	19.74138
1984	19.81494
1985	24.81166
1986	19.95917
1987	16.57604
1988	16.66731
1989	16.23356
1990	15.46839
1991	16.83088
1992	16.64042
1993	15.84045
1994	16.09216
1995	16.27698
1997	17.52181
1998	17.59165
1999	15.2805
2000	16.02931
2001	15.61951
2002	15.93024
2003	16.91443
2004	18.74799
2005	19.73541
2006	26.31019
2007	25.86676
2008	27.09401
2009	27.42707
2010	22.95753
2011	23.08748
2012	20.92865
2013	17.12826
2014	16.73054
2015	17.00022
2017	17.19803