Year	Number
1971	36.50862
1972	36.47457
1973	36.64014
1974	36.75119
1975	37.21392
1976	37.12532
1977	37.65171
1978	37.91506
1979	36.505
1980	38.14378
1981	39.1241
1982	37.19168
1983	36.44087
1984	36.32986
1985	35.8937
1986	35.01423
1987	34.31584
1988	32.0807
1989	30.4227
1990	27.66815
1991	25.22939
1992	23.52934
1993	23.15899
1994	23.76735
1995	24.59969
1996	24.68461
1997	23.68339
1998	22.6941
1999	22.12852
2000	21.02036
2001	19.90796
2002	18.2354
2003	17.68318
2004	17.80924
2005	18.10403
2006	18.03688
2007	18.10598
2008	18.05288
2009	18.01945
2010	17.63652
2011	16.2053
2012	15.97947
2013	15.61105
2014	14.91446
2015	14.40724
2016	13.84298
2017	13.26544