Year	Number
1972	23.21548
1973	23.60389
1974	24.11013
1975	24.41414
1976	24.70403
1977	24.99017
1978	24.95231
1979	25.12209
1980	25.6743
1981	26.0625
1982	24.9181
1983	25.62395
1984	25.56469
1985	26.39738
1986	25.16054
1987	24.38806
1988	23.4705
1989	23.16933
1990	23.40616
1991	23.42582
1992	23.39353
1993	23.49348
1994	23.79242
1995	24.63143
1996	24.76143
1997	25.02606
1998	25.32294
1999	25.38925
2000	25.13677
2001	25.14492
2002	25.57802
2003	25.4306
2004	26.51402
2005	26.72633
2006	25.63662
2007	24.92532
2008	23.98603
2009	23.51221
2010	23.57312
2011	21.58815
2012	23.50418
2013	21.95603
2014	21.39678
2015	21.00043
2016	21.49
2017	21.30319
2018	21.60546