Year	Number
1970	31.30673
1971	31.23792
1972	31.17206
1973	31.20517
1974	31.05682
1975	31.07902
1976	30.7193
1977	30.18095
1978	29.96131
1979	29.92785
1980	29.49813
1981	29.12599
1982	28.75722
1983	27.68637
1984	27.38856
1985	26.57015
1986	26.15051
1987	25.75239
1988	25.63145
1989	22.64261
1990	24.0697
1991	24.04182
1992	23.53131
1993	23.0639
1994	23.57823
1995	22.95187
1996	23.19249
1997	22.43882
1998	22.56369
1999	22.90516
2000	22.39068
2001	21.96937
2002	21.79427
2003	21.52564
2004	21.24495
2005	21.59226
2006	21.34929
2007	21.25952
2008	20.90504
2009	21.02566
2010	21.05971
2011	20.36261
2012	19.47213
2013	19.21687
2014	19.41851
2015	19.56773
2016	20.02351
2017	20.62693
2018	20.78495