Year	Number
1971	6.37975
1973	10.37577
1974	11.29022
1975	11.88664
1976	15.36952
1977	17.06051
1979	13.11905
1980	13.62146
1981	14.20721
1982	13.71362
1983	14.18778
1984	12.1403
1985	11.97466
1986	12.03069
1987	11.89538
1988	11.65859
1989	10.72607
1990	9.89289
1991	9.82787
1992	9.01337
1993	7.856
1994	7.07743
1995	6.0332
1996	5.32703
1997	4.94316
1998	4.84326
1999	6.52368
2000	6.69369
2001	7.84683
2002	7.90372
2003	5.28324
2004	5.36899
2005	5.1803
2006	5.89563
2007	6.36853
2008	6.72106
2009	6.27165
2010	5.24128
2011	7.07313
2012	6.42806
2013	5.41018
2014	4.59873
2015	4.59684
2016	3.98398
2017	4.58827
2018	4.79499