Year	Number
1971	20.22832
1972	20.76823
1973	18.80691
1974	20.60853
1975	21.04608
1976	22.7033
1977	22.99619
1978	26.19271
1979	28.29969
1980	27.19384
1981	30.26558
1988	28.74152
1989	26.94065
1990	25.85174
1991	27.66054
1992	26.57775
1996	26.10147
1997	27.29114
1998	27.33444
1999	24.38601
2000	23.40357
2001	23.54173
2002	26.89868
2003	28.62846
2004	30.86007
2005	27.38444
2006	29.48641
2010	30.8371
2011	34.67953
2013	26.76826
2014	25.11458
2015	27.95952
2016	26.73945
2017	29.73171