Year	Number
1971	5.94792
1973	6.53043
1974	7.87234
1975	6.91216
1976	6.33143
1977	5.19535
1978	5.32394
1979	5.55708
1980	5.52838
1981	5.17917
1984	5.59016
1986	6.00302
1987	4.5905
1988	4.35125
1990	5.24613
1999	13.78155
2000	7.86303
2001	9.76506
2002	12.752
2003	15.12834
2004	14.51841
2005	15.29279
2009	18.65304
2010	22.17533
2011	28.52265
2013	25.80991
2014	23.20024
2015	23.8808
2016	32.46851
2017	23.42604
2018	24.74281