Year	Number
1971	19.3187
1972	19.43494
1980	21.58619
1981	22.22249
1982	21.84876
1983	22.125
1984	22.3913
1985	22.60677
1986	22.80474
1987	22.74525
1988	22.87314
1989	22.8345
1990	23.03529
1991	22.95321
1992	20.01636
1993	15.96477
1994	17.56116
1995	18.15357
1996	19.14312
1997	18.8734
1998	19.21831
1999	19.9741
2000	21.04087
2001	20.9017
2002	21.98123
2003	21.67022
2004	21.17837
2005	20.52241
2006	20.26414
2007	20.08508
2008	19.64409
2009	19.10484
2010	18.0777
2011	18.87546
2012	18.50724
2013	18.20031
2014	18.2537
2015	18.34831
2016	17.70366
2017	17.48363
2018	17.09409