Year	Number
1971	23.32729
1972	22.63746
1973	21.72124
1974	21.09322
1975	20.55043
1976	20.03306
1977	20.10764
1978	19.94377
1979	19.36901
1980	18.98941
1982	18.71611
1983	18.26122
1984	17.97819
1985	17.52049
1987	20.29108
1988	20.32558
1989	19.62207
1990	19.67982
1991	19.78394
1992	19.80669
1999	18.69099
2000	18.66476
2001	17.58523
2002	17.17514
2003	17.10493
2004	18.09401
2005	17.47359
2006	18.08045
2007	17.24426
2008	18.26999
2009	17.96198
2010	17.52706
2011	17.2654
2012	18.30825
2013	18.43752
2014	16.5528
2016	15.05392
2017	15.13275