Year	Number
1971	24.97572
1972	26.62731
1974	28.3238
1975	28.54113
1976	27.62266
1977	29.68933
1978	28.12015
1979	26.77595
1980	25.09147
1981	24.00977
1982	25.50856
1983	22.71702
1984	22.80883
1986	20.39983
1987	19.40773
1988	17.95311
1990	21.30897
1991	19.20263
1992	18.66078
1993	18.59011
1994	19.84048
1995	18.42119
1996	16.64192
1997	15.8205
1999	16.89777
2000	16.94733
2001	16.96453
2002	17.3925
2003	17.2801
2004	17.07949
2009	12.13411
2016	14.79149
2017	15.21374
2018	15.16281