Year	Number
1971	20.35967
1972	19.88605
1979	21.217
1980	21.56324
1981	21.82308
1982	20.88866
1983	20.77108
1984	19.56967
1986	18.43565
1987	18.60555
1988	18.876
1989	17.95351
1990	17.82362
1991	14.994
1992	15.39502
1993	15.66375
1994	16.3438
1995	16.4373
1996	16.4481
1997	17.12607
1999	16.71512
2000	16.97265
2003	18.09706
2004	18.72611
2017	20.26796
2018	19.42453