Year	Number
1973	29.30221
1974	29.15134
1975	26.80335
1976	26.94319
1977	27.41452
1978	26.97588
1979	28.10422
1980	29.23603
1981	27.95767
1982	26.64559
1983	26.38685
1984	26.10969
1985	25.88722
1986	26.17755
1987	25.66187
1988	26.12935
1989	25.90713
1990	24.68638
1991	25.2207
1992	26.72153
1993	27.45403
1994	28.04404
1995	28.1141
1996	28.46075
1997	27.70398
1998	26.52002
1999	25.61687
2000	24.8505
2001	24.24633
2002	22.89314
2003	21.81098
2004	20.5573
2005	19.7713
2006	21.63994
2007	20.75742
2008	20.15855
2009	20.46727
2010	21.62944
2011	21.36977
2012	21.73918
2013	21.16254
2014	19.71622
2015	18.96185
2016	18.40786
2017	17.60993
2018	17.16188