Year	Number
1977	31.58501
1978	27.70929
1979	26.67997
1980	28.3251
1981	26.83466
1983	23.92886
1984	27.24104
1985	28.71786
1986	29.9454
1987	28.44922
1988	29.72905
1989	28.54599
1990	26.99361
1991	26.28848
1992	26.95358
1994	24.28202
1995	24.15087
1997	19.21188
1998	19.75614
1999	23.41325
2000	23.66847
2001	23.66742
2002	23.95276
2003	23.50322
2004	22.83249
2005	24.0725
2006	21.22016
2007	17.59527
2008	21.7792
2009	22.30456
2010	21.80981
2011	22.08541
2012	22.14664
2014	21.09985
2015	20.54219
2016	20.81374
2017	20.62576
2018	20.26212