Year	Number
1972	12.76378
1973	14.08032
1974	14.56154
1975	15.90894
1976	18.18922
1977	17.04
1978	20.53067
1979	16.82671
1980	16.65775
1981	16.29168
1982	16.31532
1983	16.41973
1984	17.2105
1985	18.02806
1986	19.68521
1987	19.93708
1988	19.85287
1989	19.84176
1990	19.74009
1991	18.59086
1992	18.48811
1993	17.86962
1994	18.22212
1995	18.56878
1996	18.04363
1999	16.59356
2000	17.08823
2001	15.37175
2002	13.72381
2003	13.49063
2004	13.07306
2005	12.48062
2006	12.23597
2007	12.33594
2008	12.22614
2009	11.87901
2010	12.10374
2011	12.71799
2012	12.6389
2013	13.01222
2014	12.87913
2015	12.50899
2016	11.99039
2017	12.03248