Year	Number
1980	21.59085
1981	21.33785
1982	21.38237
1983	21.41376
1984	21.68058
1985	21.56982
1986	21.4924
1987	21.26866
1988	21.1136
1989	20.7591
1990	20.52492
1991	20.31181
1992	19.75881
1993	19.22224
1994	18.93589
1995	19.06129
1996	19.20564
1997	19.12784
1998	19.00429
1999	18.85855
2000	18.57885
2001	18.33393
2002	18.39209
2003	18.38193
2004	17.94303
2005	17.74818
2006	17.47109
2007	17.0174
2008	16.68211
2009	16.58544
2010	16.64097
2011	16.75759
2012	16.73481
2013	16.85867
2014	16.85403
2015	16.56762
2016	16.77199
2017	17.08293
2018	17.06837