Year	Number
1971	15.80584
1972	15.62673
1973	15.20766
1974	14.59609
1975	14.05784
1977	13.99688
1978	15.306
1979	13.66129
1980	13.53336
1981	13.2317
1982	13.03262
1983	11.84622
1984	11.83076
1999	12.221
2000	12.74229
2001	13.1808
2002	12.77342
2003	12.22628
2004	10.15341
2005	10.25668
2006	9.8669
2007	9.79414
2008	10.03077
2009	9.88786
2010	9.90598
2011	9.50424
2012	9.26577
2013	12.70659
2014	12.75811
2016	13.24117
2017	13.57979