Year	Number
1981	27.94884
1986	26.52823
1989	23.41118
1990	22.75625
1991	22.34118
1992	21.3168
1993	20.25195
1994	19.58987
1999	18.37728
2000	17.58649
2001	16.99631
2002	16.95644
2003	16.63307
2005	16.74517
2006	17.14697
2007	17.12104
2008	17.44699
2009	18.06296
2012	19.59062
2013	19.82861
2014	19.80802
2015	20.14375
2016	20.61869
2017	21.25883