Year	Number
1971	19.7045
1972	19.48533
1975	26.51605
1976	24.8675
1977	24.22518
1978	23.89035
1979	24.17733
1980	24.11576
1981	23.37655
1982	20.49073
1983	18.65688
1984	17.24961
1985	17.35831
1986	17.41592
1987	17.8308
1988	17.07252
1989	16.33143
1990	14.62755
1991	13.64396
1992	13.64034
1993	14.33091
1994	14.16307
1995	13.22695
1996	13.26022
1997	13.31432
1998	13.16764
2000	12.43361
2001	12.0058
2003	15.59586
2005	17.11212
2006	17.18416
2007	17.56221
2008	17.09274
2009	16.92128
2010	16.97089
2011	14.68572
2012	15.07155
2013	14.99331
2014	14.91572
2017	12.9705