Year	Number
1970	21.84925
1971	21.87628
1972	21.61258
1973	22.00216
1974	21.96875
1975	21.96517
1976	21.89967
1977	21.43726
1978	21.29302
1979	20.53464
1980	20.14684
1981	19.77192
1982	19.5513
1983	19.35543
1984	19.15044
1985	18.86942
1986	18.73987
1987	18.55618
1988	18.29652
1989	18.03836
1990	17.92511
1991	17.7346
1992	17.82194
1993	17.74583
1994	17.49111
1995	17.60494
1996	17.65655
1997	17.36301
1998	16.77073
1999	16.10815
2000	15.88692
2001	15.64473
2002	15.36163
2003	13.98853
2004	14.14157
2005	14.14158
2006	13.75603
2007	13.44629
2008	13.18345
2009	13.17141
2010	13.11672
2011	13.08211
2012	12.96264
2013	12.68512
2014	12.59773
2015	12.33976
2016	12.23493
2017	12.04973
2018	11.98251