Year	Number
1971	30.93285
1972	30.87426
1973	32.23596
1974	30.97766
1975	30.4632
1976	30.23067
1977	28.74069
1978	26.9149
1979	25.81226
1980	24.8178
1981	24.13617
1982	23.49298
1983	23.85878
1984	24.126
1985	23.44758
1986	23.36514
1987	22.81234
1988	22.19387
1989	21.14261
1990	19.64665
1991	18.65531
1992	18.25282
1993	17.82167
1994	16.527
1995	16.09251
1996	14.96337
1997	13.93695
1998	13.60849
1999	13.53775
2000	13.41496
2001	12.72615
2002	12.52457
2003	11.94256
2004	11.25939
2005	11.06286
2006	10.53573
2007	10.28887
2012	9.19747
2013	9.49328
2014	9.44769
2015	9.64365
2016	9.27444
2017	9.38027