Year	Number
1970	29.45008
1971	29.61628
1972	28.44963
1973	29.12612
1974	29.28465
1975	29.45642
1976	29.13866
1977	28.73633
1978	28.46929
1979	28.36357
1980	27.74365
1981	27.16831
1982	26.42851
1983	25.87502
1984	25.62108
1985	25.66156
1986	25.40181
1987	25.14344
1988	24.69877
1989	24.11141
1990	23.8239
1991	23.62578
1992	23.54704
1993	23.50368
1994	23.58415
1995	23.79357
1996	23.95179
1997	24.14198
1998	24.40915
1999	24.28938
2000	23.5501
2001	22.76456
2002	22.07314
2003	20.41779
2004	20.83687
2005	20.16691
2006	19.94994
2007	19.59169
2008	19.34271
2009	19.09353
2010	18.78234
2011	18.72064
2012	18.67235
2013	18.7395
2014	18.31817
2015	18.32687
2016	18.46745
2017	18.47165
2018	18.39381