Year	Number
1978	22.04479
1979	22.2324
1980	22.36223
1981	22.38151
1982	22.50004
1983	22.5535
1984	23.25003
1985	23.63564
1986	23.22739
1987	23.17466
1988	22.76614
1989	22.48852
1990	22.52747
1991	21.80486
1992	21.78523
1993	22.17722
1994	21.98583
1995	21.94137
1996	21.25685
1997	20.86498
1998	20.32004
1999	19.97434
2000	18.84343
2001	19.13404
2002	19.4646
2003	19.03418
2004	18.99457
2005	18.65802
2006	19.22855
2007	17.95662
2008	19.8212
2009	21.42392
2010	20.25238
2011	19.19762
2012	19.019
2013	19.25221
2014	19.58133
2015	19.78805
2016	20.45652
2017	20.49601
2018	21.16757