Year	Number
1970	26.25162
1971	25.68968
1972	25.8643
1973	25.92807
1974	26.12886
1975	27.22153
1976	27.25505
1977	27.41876
1978	27.00834
1979	27.54703
1980	26.83688
1981	26.38668
1982	26.68955
1983	26.58271
1984	25.81757
1985	25.4649
1986	25.6214
1987	24.74628
1988	24.18109
1989	23.8393
1990	22.97027
1991	22.13139
1992	22.42381
1993	22.26074
1994	21.51813
1995	21.20767
1996	20.70265
1997	20.2328
1998	21.98091
1999	20.47181
2000	20.57285
2001	19.54788
2002	20.37339
2003	18.74286
2004	19.95273
2005	19.51688
2006	20.23405
2007	19.83521
2008	19.81291
2009	19.36458
2010	18.8308
2011	18.21863
2012	17.71971
2013	20.92071
2014	20.9446
2015	20.69622
2016	20.63043
2017	20.54557
2018	20.4856