Year	Number
1972	23.21681
1973	23.60555
1974	24.11159
1975	24.41605
1976	24.70622
1977	24.99343
1978	24.95652
1979	25.12787
1980	25.68127
1981	26.07079
1982	24.92608
1983	25.63313
1984	25.5741
1985	26.4082
1986	25.1704
1987	24.39728
1988	23.47898
1989	23.17798
1990	23.41562
1991	23.43557
1992	23.40384
1993	23.50421
1994	23.8036
1995	24.64225
1996	24.77113
1997	25.03437
1998	25.3299
1999	25.39662
2000	25.14307
2001	25.15046
2002	25.58487
2003	25.43669
2004	26.52047
2005	26.73213
2006	25.64195
2007	24.92966
2008	23.98915
2009	23.51559
2010	23.57636
2011	21.59074
2012	23.50709
2013	21.95824
2014	21.39848
2015	21.00242
2016	21.49227
2017	21.30533
2018	21.60801