Year	Number
1971	18.41573
1972	17.05155
1973	17
1974	17.09091
1975	15.86916
1976	14.58621
1977	14.21368
1978	12.29545
1979	12.0303
1980	11.62879
1981	10.4069
1982	9.3
1983	9.10366
1984	8.82424
1985	8.42105
1986	8.93038
1987	7.97076
1988	7
1989	6.69841
1990	6.16583
1991	5.82692
1992	5.50459
1993	5.38462
1994	5.3242
1996	5.22581
1997	5.29412
2000	5.36052
2004	6.33772
2008	6.34274
2009	6.22222
2010	6.46311
2011	6.40574
2012	6.27092
2018	6.93061