Year	Number
1976	21.41625
1977	21.13815
1978	21.16994
1979	20.23436
1980	20.52941
1981	20.69573
1982	21.86652
1983	21.48965
1984	22.4514
1985	22.36234
1986	22.223
1987	22.42675
1988	22.36312
1989	22.24312
1990	22.3775
1991	21.72893
1992	22.01232
1993	21.93831
1994	21.44872
1995	21.75059
1996	21.0475
1997	20.79064
1998	20.52912
1999	20.12945
2000	18.94445
2001	19.72446
2002	20.77714
2003	19.95527
2004	20.54658
2005	20.26388
2006	20.73924
2007	20.66026
2008	21.87585
2009	22.70673
2010	22.40291
2011	20.84807
2012	19.59445
2013	19.96359
2014	20.94236
2015	21.31302
2016	22.25814
2017	22.49575
2018	22.61583