Year	Number
1970	38.20769
1971	38.3984
1972	34.56653
1973	33.91222
1974	32.8813
1975	32.07068
1976	32.25021
1977	32.37915
1978	32.50871
1979	31.14374
1980	30.56291
1981	30.62569
1982	29.83356
1983	29.73447
1984	29.60797
1985	30.38614
1986	29.44692
1987	29.78139
1988	29.84056
1989	29.89499
1990	29.91953
1991	30.10601
1992	27.86149
1993	27.6851
1994	25.85626
1995	24.8125
1996	25.35665
1998	23.29196
1999	24.02045
2000	26.45241
2001	26.01713
2002	26.81989
2004	27.95172
2005	28.34839
2006	28.18322
2007	29.51974
2008	29.3957
2009	29.30908
2010	28.13107
2011	27.57965
2012	25.04796
2013	24.98364
2014	24.28676
2015	24.23937
2016	24.31808
2017	23.60132
2018	23.3015