Year	Number
1971	19.73254
1972	19.66533
1973	19.76722
1974	19.86462
1975	20.2222
1976	20.29274
1977	20.66347
1978	21.01243
1979	21.12709
1980	21.04431
1981	20.41042
1982	20.49197
1983	20.57938
1984	20.66559
1985	20.69347
1986	20.63869
1987	20.62745
1988	20.81457
1989	20.6611
1990	21.23902
1991	21.07203
1992	21.09149
1993	21.02188
1994	21.2948
1995	21.49567
1996	21.34217
1997	21.29818
1998	21.96231
1999	22.19825
2000	21.92308
2001	22.36814
2002	22.54164
2003	22.59947
2004	22.75251
2005	22.9992
2006	23.27465
2007	22.91328
2008	23.20479
2009	22.93793
2010	22.87612
2011	22.72036
2012	22.90115
2013	22.42411
2014	22.50288
2015	22.10181
2016	21.97853
2017	21.88316
2018	22.09153