Year	Number
1970	17.29139
1971	18.03585
1972	18.87785
1973	20.58821
1974	21.50076
1975	23.83082
1976	25.75911
1977	27.54538
1978	29.29366
1981	24.73582
1982	24.94844
1983	21.3093
1984	21.585
1985	20.82346
1986	21.89817
1987	20.99217
1988	20.46028
1989	19.99637
1990	19.68698
1991	19.68194
1992	19.44669
1993	20.2281
1994	18.21072
1995	19.03209
1996	18.75551
1997	18.41598
1998	17.22606
1999	21.62919
2000	20.35754
2001	20.31555
2002	18.91347
2003	16.18434
2004	16.636
2005	16.72571
2006	16.21431
2007	16.388
2008	16.23886
2009	16.53146
2010	15.99044
2011	16.69359
2012	16.75243
2013	15.46321
2014	14.20135
2015	14.09055
2016	14.33983
2017	14.57939
2018	14.20042