Year	Number
1970	22.04624
1971	22.03838
1972	21.65426
1973	21.44032
1974	21.53651
1975	21.66355
1976	21.91791
1977	22.31925
1978	22.60394
1979	23.02306
1980	23.59884
1981	23.92661
1982	24.42115
1983	24.86232
1984	25.26786
1985	25.8411
1986	26.26825
1987	26.63441
1988	27.02872
1989	27.44692
1990	27.69485
1991	28.19097
1992	28.67432
1993	29.30353
1994	29.84336
1995	30.39654
1996	30.8882
1997	31.41292
1998	31.84932
1999	32.33524
2000	32.47387
2001	32.35375
2002	31.25232
2003	31.09285
2004	31.12799
2005	29.3956
2006	28.3779
2007	27.29504
2008	26.36226
2009	25.23992
2010	25.37949
2011	25.9675
2012	25.91948
2013	29.92496
2014	30.60243
2015	30.44821
2016	28.20535
2017	26.97132
2018	27.19053