Year	Number
1973	18.39456
1974	17.7482
1976	15.36207
1977	17.3423
1980	17.97448
1981	17.96963
1982	18.866
1983	21.06061
1984	18.70531
1985	19.14216
1986	24.37963
1987	23.34375
1988	21.88594
1989	21.28195
1990	17.93651
1991	16.44331
1992	17.00867
1993	17.25251
1994	16.54398
1995	14.55366
1996	13.65973
1997	13.47527
1998	12.81566
1999	12.55168
2000	10.99562
2001	10.58817
2002	11.09007
2003	10.69406
2004	11.16117
2005	9.78375
2006	12.02475
2007	11.8019
2008	14.31208
2009	11.28911
2010	14.16805
2011	12.42898
2012	12.67639
2013	13.04176
2014	13.19023
2015	12.60014
2016	12.06254
2017	11.78786