Year	Number
1983	22.3579
1984	22.83104
1985	23.64048
1986	22.9982
1987	22.3281
1988	23.38595
1989	24.07015
1990	24.93444
1991	26.0425
1992	26.76539
1993	27.67611
1994	23.08985
1995	23.19476
1996	25.2365
1999	24.60828
2000	23.57457
2001	22.43671
2002	23.08717
2003	21.93834
2004	21.59752
2005	21.35188
2006	20.37274
2007	19.47541
2008	19.49843
2009	18.23624
2010	18.66206
2011	19.01621
2012	19.41051
2013	19.02737
2014	18.9205
2015	20.3005
2016	20.28315
2017	20.12302