Year	Number
1971	15.44157
1978	19.89001
1980	20.28531
1991	14.65205
1999	28.40924
2000	28.97326
2007	23.9792
2008	23.54798
2009	23.409
2011	24.71884
2013	19.2752
2014	18.88774
2015	19.19297
2016	19.4834
2017	17.43224