Year	Number
1970	32.01087
1971	31.94643
1972	31.86829
1973	31.82907
1974	31.49153
1975	31.28897
1976	31.21494
1977	31.07388
1978	30.88713
1979	30.62616
1980	30.48228
1981	30.31727
1982	29.86419
1983	29.51284
1984	29.21674
1985	28.93145
1986	28.33088
1987	28.04448
1988	27.667
1989	27.32523
1990	27.20317
1991	27.05967
1992	26.7375
1993	26.40943
1994	25.99278
1995	25.8661
1996	25.54744
1997	25.29354
1998	25.58835
1999	25.96649
2000	25.44362
2001	24.48454
2002	24.07003
2003	23.73337
2004	23.89112
2005	23.61684
2006	23.76778
2007	24.10191
2008	23.75326
2009	23.64576
2010	23.28145
2011	22.94251
2012	22.44992
2013	22.52229
2014	22.25158
2015	21.98897
2016	21.84299
2017	21.61644
2018	21.57