Year	Number
1971	40.00059
1975	41.96422
1976	40.94596
1977	39.50841
1978	37.58111
1979	36.76106
1980	35.80454
1981	35.24855
1982	33.74068
1983	32.32614
1984	30.56192
1985	29.63073
1986	27.84273
1987	27.28204
1988	27.17892
1989	27.95506
1990	27.78717
1991	27.69468
1992	28.1692
1993	27.41847
1994	27.36828
1995	27.27589
1996	27.32222
1997	27.34591
1999	28.19076
2000	28.3962
2001	27.84252
2002	27.59291
2003	27.53299
2004	26.51107
2005	25.43721
2006	24.48384
2007	23.93164
2008	23.23052
2009	22.99987
2010	23.32803
2011	23.26903
2012	23.15942
2013	23.26105
2014	23.66406
2015	23.83613
2016	24.2355
2017	24.22505
2018	24.32018