Year	Number
1971	17.19386
1972	16.15668
1973	15.75477
1974	15.83229
1975	17.31067
1976	17.06804
1977	16.8216
1978	19.11541
1979	18.41028
1980	15.36629
1982	15.06617
1983	14.11279
1985	14.85322
1986	9.35837
1987	10.85239
1988	12.29803
1989	13.49941
1990	13.37445
1991	13.52176
1992	14.23678
1993	12.06916
2001	15.33285
2002	15.13902
2005	13.90109
2006	14.07804
2007	14.00385
2009	13.30544
2010	13.84981
2011	13.42443
2012	12.47988
2013	12.75306
2014	12.32072
2015	11.97658