Year	Number
1978	9.26786
1979	13.35526
1980	13.75652
1981	11.1323
1982	12.8566
1984	19.26667
1985	16.80392
1986	17.46517
1987	19.00696
1988	16.75494
1989	16.01464
1990	13.69516
1991	12.12366
1992	9.61645
2009	18.57074
2011	19.12622
2012	18.39752
2013	17.6851
2014	17.46125
2015	17.80907
2016	16.96831
2017	18.41871