Year	Number
1970	16.54511
1972	19.43271
1975	22.62893
1976	19.61741
1979	18.3438
1980	18.97862
1981	19.25363
1982	17.83865
1983	17.2838
1985	17.01663
1986	17.72376
1987	18.06675
1988	17.66259
1989	18.4735
1990	18.95094
1991	19.21686
1992	19.82895
1993	19.39864
1994	19.19469
1995	19.23877
1996	18.03308
1997	18.38004
1998	18.95822
1999	19.5273
2000	18.84161
2001	19.57279
2002	18.75178
2006	17.79432
2007	17.91364
2008	15.64106
2009	15.81656
2010	15.5078
2011	14.92131
2013	13.96964
2014	14.35946
2015	13.9521
2016	12.73154
2017	13.31491
2018	12.42779