Year	Number
1971	8.56418
1972	8.70034
1973	8.68096
1974	8.42732
1975	8.01955
1981	9.70992
1982	9.94488
1983	10.30641
1984	10.81586
1985	11.56035
1986	12.07073
1994	9.19634
1995	9.21942
1996	9.08052
1997	9.13007
2007	10.9478
2008	10.23001
2009	9.93713
2010	8.26347
2011	9.13457
2012	8.19896
2013	7.28747
2014	7.16789
2015	6.99524
2016	7.37407
2017	7.18354