Year	Number
1970	31.88262
1971	31.81208
1972	31.69956
1973	31.67002
1974	31.33282
1975	31.15223
1976	31.09063
1977	30.94839
1978	30.76319
1979	30.50962
1980	30.34631
1981	30.15168
1982	29.72465
1983	29.36621
1984	29.10998
1985	28.84569
1986	28.22255
1987	27.9513
1988	27.6014
1989	27.26753
1990	27.14855
1991	27.01075
1992	26.69325
1993	26.37316
1994	25.99206
1995	25.85904
1996	25.55242
1997	25.33853
1998	25.60389
1999	25.93376
2000	25.42718
2001	24.50618
2002	24.10934
2003	23.78552
2004	23.82575
2005	23.5294
2006	23.65543
2007	23.90935
2008	23.56049
2009	23.3968
2010	23.06108
2011	22.71553
2012	22.24679
2013	22.24736
2014	21.96339
2015	21.72139
2016	21.55642
2017	21.34033
2018	21.30215