Year	Number
1971	19.57274
1981	13.6875
1982	12.89659
1983	12.52444
1984	11.80202
1985	11.74935
1986	11.44632
1987	10.88133
1988	10.53639
1989	10.14936
1990	9.96332
1991	9.7814
1992	9.68867
1993	9.486
1994	9.40802
1995	9.40288
1996	9.87243
1997	9.94267
1998	10.15429
1999	10.30091
2002	10.43735
2003	10.52508
2004	10.70672
2005	10.98323
2006	10.94344
2007	10.84635
2008	10.57966
2009	10.28249
2010	9.9725
2011	9.67872
2012	9.53847
2013	9.67333
2014	9.61071
2015	9.57663
2016	9.41777
2017	9.32983