Year	Number
1971	20.69884
1972	20.7006
1973	20.47527
1974	20.25065
1975	20.02003
1976	19.78985
1977	19.55744
1978	19.32768
1979	19.08665
1980	18.87184
1981	18.63465
1982	18.3975
1983	18.15904
1984	17.90409
1985	17.36699
1986	17.10132
1987	16.83791
1988	16.56282
1989	16.32931
1990	16.07617
1991	15.80949
1992	15.59185
1993	15.74379
1994	15.91407
1995	15.69838
1996	15.69789
1997	15.59646
1998	15.49327
1999	15.23733
2000	14.93725
2001	15.45445
2002	15.19328
2003	15.2173
2004	15.27175
2005	15.23872
2006	14.93628
2007	14.88559
2008	14.71032
2009	14.31685
2010	14.12463
2011	14.81487
2012	14.99648
2013	15.02165
2014	15.10117
2015	15.01378
2016	14.9758
2017	14.94246
2018	14.93963