Year	Number
1973	17.1562
1974	17.41474
1975	17.48101
1976	18.4077
1977	19.77253
1978	18.8235
1979	18.82538
1980	19.23378
1981	19.87755
1982	19.5332
1983	19.48647
1984	19.37228
1985	19.21422
1986	19.26908
1987	19.31649
1988	19.48962
1989	19.57137
1990	19.12027
1991	18.9155
1992	19.30397
1993	19.91587
1994	19.99508
1995	20.1331
1996	20.20103
1997	19.95153
1998	20.07262
1999	18.94722
2000	17.78334
2001	18.1343
2002	17.67355
2003	17.17749
2004	17.05457
2005	16.54497
2006	17.41719
2007	17.37194
2008	17.36547
2009	17.80508
2010	18.10701
2011	18.30909
2012	18.6467
2013	19.16339
2014	18.42788
2015	18.27242
2016	18.24748
2017	17.86705
2018	17.63613