Year	Number
1970	32.91612
1971	32.84191
1972	32.49413
1973	33.345
1974	32.12982
1975	32.24263
1976	31.50185
1977	30.84299
1978	31.78403
1979	31.37767
1980	30.67144
1981	30.16444
1982	29.43576
1983	26.10093
1984	25.95679
1985	25.46184
1986	25.56636
1987	26.95056
1988	27.2168
1989	24.4286
1990	26.29986
1991	26.49598
1992	26.17547
1993	25.85368
1994	26.38096
1995	25.87607
1996	26.31954
1997	25.25473
1998	25.21488
1999	24.87611
2000	24.34276
2001	23.83349
2002	23.67198
2003	23.31166
2004	22.9917
2005	23.39561
2006	23.13481
2007	23.04397
2008	22.83999
2009	22.69942
2010	23.0273
2011	22.35698
2012	21.39288
2013	21.89897
2014	22.03949
2015	22.25553
2016	22.63736
2017	23.17948
2018	23.00915