Year	Number
1971	33.08076
1972	29.83502
1973	32.60536
1974	31.26956
1975	30.15065
1976	28.69082
1977	24.92326
1978	23.86802
1979	19.93524
1980	18.53022
1981	17.27748
1982	16.74142
1983	16.28378
1984	15.67284
1985	15.23439
1986	15.35334
1987	14.89609
1988	14.49178
1989	14.1643
1990	14.29621
1991	14.33181
1992	14.99184
1993	14.99127
1994	14.34927
1995	13.76533
1996	13.85412
1998	13.37752
1999	13.23326
2000	13.01974
2001	12.77769
2002	12.71834
2003	12.0928
2004	12.4081
2005	12.31632
2006	12.37337
2007	11.6213
2013	12.70434
2014	12.53097
2015	12.40968
2016	12.03926
2017	11.61756