Year	Number
1971	22.56676
1972	22.33953
1973	22.6691
1974	22.90855
1975	23.16941
1976	22.99202
1977	22.66492
1978	22.51177
1979	22.17642
1980	21.83763
1981	21.50597
1982	21.16485
1983	20.86651
1984	20.62099
1985	20.49116
1986	20.44565
1987	20.31342
1988	20.08634
1989	19.81797
1990	19.56474
1991	19.32997
1992	19.49262
1993	19.58287
1994	19.48514
1995	19.50591
1996	19.74796
1997	19.51213
1998	20.2181
1999	19.41894
2000	18.97004
2001	18.52362
2002	17.68886
2003	17.06123
2004	16.50999
2005	16.70221
2006	16.2511
2007	16.03527
2008	16.04265
2009	15.67872
2010	15.06225
2011	14.60058
2012	14.14301
2013	14.08836
2014	14.03277
2015	14.11338
2016	14.33638
2017	14.31793
2018	14.21111