Year	Number
1970	25.63793
1971	24.64984
1972	24.37463
1973	24.14731
1974	23.12903
1975	22.51194
1976	21.89086
1977	21.19494
1978	17.95674
1979	17.85567
1980	18.74815
1981	18.36386
1982	17.72535
1983	18.35109
1984	18.37324
1985	18.01176
1986	17.67955
1987	18.23465
1988	17.35435
1989	16.80983
1990	17.24678
1992	17.41283
1994	19.1609
1995	19.29128
1996	17.64643
1997	17.00685
1998	16.15214
2001	18.40479
2002	15.35487
2003	13.86223
2004	13.22773
2005	13.59045
2006	12.07948
2007	11.93888
2008	11.7383
2010	10.94397
2011	10.23401
2012	10.17317
2013	9.75621
2014	10.27401
2015	9.09392
2016	10.09107