Year	Number
1970	26.35164
1971	25.72848
1972	25.85893
1973	25.90457
1974	25.83653
1975	26.86166
1976	27.03707
1977	27.13498
1978	26.74137
1979	27.13444
1980	26.44428
1981	26.08876
1982	26.48105
1983	26.45542
1984	25.73784
1985	25.42328
1986	25.64252
1987	24.80891
1988	24.31804
1989	23.95523
1990	23.03963
1991	22.17009
1992	22.44681
1993	22.26161
1994	21.4922
1995	21.15785
1996	20.64768
1997	20.18649
1998	21.96197
1999	20.41032
2000	20.55207
2001	19.4843
2002	20.33127
2003	18.68348
2004	19.98791
2005	19.52438
2006	20.29478
2007	19.93233
2008	19.86734
2009	19.36692
2010	18.76336
2011	18.06307
2012	17.5225
2013	20.75762
2014	20.79113
2015	20.57204
2016	20.53344
2017	20.46103
2018	20.40106