Year	Number
1970	30.98008
1975	31.90474
1977	31.50006
1978	30.93067
1979	30.90308
1980	27.27211
1981	26.38464
1982	25.97114
1983	25.96064
1984	25.65363
1985	24.05382
1987	22.221
1988	21.34314
1989	21.37015
1990	20.37696
1991	20.14146
1992	20.12766
1993	20.01525
1994	19.69954
1995	19.93694
1996	19.41584
1998	21.80374
1999	20.36262
2000	19.55776
2001	19.66518
2002	18.91971
2003	17.51402
2004	17.48584
2005	16.89527
2006	15.65511
2007	14.98441
2008	14.44237
2009	13.13521
2010	12.60115
2011	12.32447
2012	12.00361
2013	11.83126
2014	11.4126
2015	11.521
2016	11.61165
2017	11.6556