Year	Number
1970	31.02171
1971	30.9578
1972	30.6511
1973	31.34774
1974	30.32097
1975	30.40108
1976	29.7017
1977	29.09745
1978	29.83235
1979	29.43446
1980	28.83321
1981	28.30539
1982	27.74397
1983	24.98445
1984	24.86562
1985	24.42707
1986	24.51193
1987	25.65096
1988	25.90237
1989	23.47212
1990	24.95574
1991	25.05527
1992	24.64181
1993	24.23943
1994	24.58392
1995	24.06612
1996	24.35856
1997	23.48136
1998	23.38246
1999	23.06245
2000	22.56739
2001	22.06592
2002	21.79525
2003	21.38683
2004	21.00318
2005	21.20346
2006	20.98418
2007	20.86363
2008	20.53933
2009	20.52897
2010	20.67443
2011	20.10857
2012	19.36407
2013	19.58594
2014	19.74702
2015	19.94048
2016	20.45241
2017	21.13095
2018	21.32469