Year	Number
1970	30.7062
1971	30.86438
1972	30.09588
1973	30.73709
1974	30.81381
1975	30.92383
1976	30.76877
1977	30.87723
1978	30.75317
1979	30.67952
1980	30.1961
1981	29.73872
1982	29.12932
1983	28.58061
1984	28.32915
1985	28.20665
1986	27.93487
1987	27.68928
1988	27.36779
1989	26.79544
1990	26.70416
1991	26.60429
1992	26.58135
1993	26.58624
1994	26.71538
1995	26.75868
1996	26.90631
1997	26.93566
1998	27.02443
1999	26.96506
2000	26.50355
2001	25.98967
2002	25.56539
2003	24.67268
2004	24.81289
2005	24.49458
2006	24.25543
2007	24.03075
2008	23.92234
2009	23.46273
2010	23.04905
2011	22.78424
2012	22.65163
2013	22.03846
2014	21.65103
2015	21.50841
2016	22.06742
2017	21.98289
2018	21.996