Year	Number
1980	24.82772
1981	24.50775
1982	24.63465
1983	24.76784
1984	25.20943
1985	25.11435
1986	25.03414
1987	24.69148
1988	24.43787
1989	23.89416
1990	23.50214
1991	23.16886
1992	22.35042
1993	21.60117
1994	21.15052
1995	21.22289
1996	21.30444
1997	21.19213
1998	21.00838
1999	20.77612
2000	20.40795
2001	20.06541
2002	19.81363
2003	19.53694
2004	19.05629
2005	18.98538
2006	18.71192
2007	18.29529
2008	18.07009
2009	18.05049
2010	18.19861
2011	18.00801
2012	17.96262
2013	18.11481
2014	18.09482
2015	17.82266
2016	18.15291
2017	18.41218
2018	18.39211