Year	Number
1977	33.43674
1978	28.12366
1979	26.4501
1980	27.13053
1981	27.70114
1982	24.27663
1983	23.30159
1985	26.02642
1986	25.89368
1987	23.79298
1990	22.21037
1991	23.46942
1992	21.4341
1993	21.94805
1994	24.96983
1995	26.56277
1996	25.74314
1997	24.79502
1998	23.78148
1999	23.1816
2000	22.05068
2001	21.43757
2002	20.5693
2003	20.76639
2004	20.39125
2011	19.31548
2012	19.08136
2013	18.87107
2014	17.63441
2015	17.42302
2016	17.13565
2017	17.58267
2018	17.27017