Year	Number
1971	13.80515
1972	14.84428
1973	15.14367
1974	15.26902
1975	15.96879
1976	16.36202
1977	16.29473
1978	16.58448
1979	13.9195
1980	17.0382
1981	16.97743
1982	17.01412
1983	16.49681
1984	16.04356
1985	16.03754
1986	15.37124
1987	15.07604
1988	14.41024
1993	12.98818
1996	11.9574
1998	11.55459
2000	13.57916
2001	13.57838
2002	13.57727
2003	13.27407
2004	13.34317
2005	13.90459
2006	14.36216
2007	14.65735
2009	20.52053
2010	19.9457
2011	20.90102
2012	21.96425
2013	23.0055
2014	23.03294
2015	22.35071
2016	21.94529
2017	20.99927
2018	20.62112