Year	Number
1970	29.72213
1971	29.88888
1972	28.55831
1973	29.27945
1974	29.43986
1975	29.63296
1976	29.47242
1977	29.53323
1978	29.27433
1979	29.19023
1980	28.51215
1981	27.94111
1982	27.16429
1983	26.58446
1984	26.07273
1985	25.98704
1986	25.56154
1987	25.15158
1988	24.57088
1989	24.00196
1990	23.51734
1991	23.3779
1992	23.41051
1993	23.45646
1994	23.5636
1995	23.70778
1996	23.93948
1997	24.15827
1998	24.4615
1999	24.33867
2000	23.821
2001	23.16444
2002	22.35288
2003	20.43202
2004	20.82951
2005	20.21877
2006	19.6508
2007	19.15573
2008	19.22901
2009	18.76306
2010	18.40429
2011	18.29799
2012	18.24173
2013	17.73742
2014	17.43074
2015	17.20192
2016	16.96202
2017	17.41387
2018	17.53589