Year	Number
1970	31.79074
1971	31.63676
1972	31.42649
1973	29.33498
1974	27.37374
1975	26.00121
1976	24.67938
1977	23.14471
1978	21.7755
1979	19.82412
1980	20.18467
1981	20.40705
1982	20.68609
1983	21.11838
1984	20.94215
1985	21.81612
1986	22.71182
1987	22.16021
1988	21.88242
1989	21.8104
1990	21.12971
1991	21.233
1992	20.68527
1993	21.16726
1994	22.46343
1995	23.9235
1996	23.89051
1997	24.44404
1998	25.76604
1999	26.13844
2000	26.11761
2001	24.9275
2002	25.19635
2003	23.06335
2004	21.98676
2005	22.3399
2006	21.68399
2007	21.50505
2008	21.66005
2009	21.6212
2010	21.46053
2011	20.35924
2012	20.93864
2013	19.79263
2014	18.73281
2015	18.82093
2016	17.79444
2017	17.93769
2018	16.19841