Year	Number
1971	30.41958
1972	31.21418
1975	30.88465
1980	31.52744
1981	32.29537
1983	31.93617
1986	30.23302
1987	30.87017
1988	30.70039
1989	30.11647
1990	29.8018
1991	28.11374
1992	24.81348
1993	22.45483
1994	22.56422
1995	23.71569
1996	24.83578
1997	30.86767
1998	31.67076
1999	32.44852
2000	32.58434
2001	32.29349
2002	31.78211
2003	30.76911
2004	32.8681
2005	34.23344
2006	33.01005
2007	31.59826
2008	31.14934
2009	30.36106
2010	30.23907
2011	29.32321
2012	28.77793
2013	27.58707
2014	27.20739
2015	28.222
2016	29.69343
2017	30.37955
2018	30.38318