Year	Number
1970	27.90849
1971	28.18103
1972	27.07751
1973	27.84562
1974	28.15748
1975	28.40196
1976	28.18799
1977	27.82323
1978	27.48829
1979	27.42133
1980	26.86667
1981	26.37828
1982	25.60934
1983	25.26582
1984	24.93673
1985	24.94702
1986	24.6777
1987	24.26641
1988	23.74697
1989	23.10312
1990	22.7473
1991	22.54582
1992	22.41292
1993	22.40899
1994	22.57163
1995	22.87334
1996	23.08516
1997	23.30859
1998	23.57319
1999	23.43892
2000	22.77819
2001	21.9088
2002	21.2232
2003	19.35308
2004	19.74963
2005	19.04118
2006	18.80967
2007	18.42971
2008	18.13589
2009	17.81222
2010	17.52474
2011	17.43371
2012	17.37319
2013	17.36981
2014	16.91172
2015	16.96478
2016	17.14734
2017	17.2078
2018	17.16688