Year	Number
1972	21.75419
1973	21.77168
1974	21.81572
1975	21.94326
1976	21.81616
1977	21.92925
1978	21.9748
1979	22.00413
1980	22.3074
1981	22.1402
1982	22.37754
1983	22.49492
1984	22.4239
1985	22.49408
1986	22.07274
1987	21.68728
1988	20.97851
1989	20.87558
1990	20.85305
1991	21.0574
1992	21.51963
1993	21.76452
1994	22.40389
1995	23.17625
1996	23.82334
1997	24.43975
1998	25.3594
1999	26.08005
2000	26.07487
2001	26.03394
2002	25.62723
2003	24.88508
2004	24.32356
2005	23.51519
2006	23.84524
2007	23.57327
2008	23.95458
2009	23.95345
2010	23.83878
2011	24.0673
2012	24.15077
2013	24.12494
2014	24.09625
2015	23.66721
2016	23.4596
2017	23.10511
2018	23.53348