Year	Number
1970	32.16862
1971	32.21477
1972	31.48467
1973	32.12838
1974	32.17586
1975	32.23763
1976	32.07494
1977	32.16311
1978	31.91507
1979	31.88305
1980	31.60248
1981	31.27037
1982	30.75483
1983	30.21281
1984	29.9472
1985	29.84885
1986	29.56695
1987	29.43037
1988	29.21705
1989	28.64245
1990	28.62723
1991	28.58179
1992	28.54211
1993	28.55228
1994	28.73237
1995	28.77893
1996	28.79918
1997	29.01088
1998	29.13424
1999	29.12374
2000	28.91662
2001	28.54492
2002	28.41435
2003	27.63181
2004	27.84555
2005	27.7224
2006	27.63726
2007	27.55731
2008	27.50847
2009	27.11224
2010	26.70045
2011	26.36668
2012	26.42209
2013	25.56558
2014	25.24869
2015	25.04042
2016	25.49018
2017	25.39491
2018	25.44687