Year	Number
1971	30.78901
1972	26.01015
1981	28.27108
1982	28.15021
1983	27.20275
1984	26.58332
1986	25.28136
1987	25.50205
1988	24.71025
1990	23.48296
1991	22.02928
1992	21.15976
1993	19.99801
1994	19.2214
1995	17.83949
1998	15.47547
1999	15.04197
2000	14.54434
2001	14.30441
2002	14.00892
2003	13.87611
2004	13.93149
2005	13.75078
2006	13.58265
2007	12.8265
2008	12.42388
2009	12.62216
2010	12.40145
2011	12.39876
2012	12.60121
2013	13.31975
2014	13.09629
2015	13.18623
2016	12.99102
2017	13.13172