Year	Number
1970	20.40194
1971	20.95485
1972	21.6182
1973	20.75158
1974	20.15272
1975	20.30488
1976	20.97378
1977	21.032
1978	20.90133
1979	20.38001
1980	19.44001
1981	18.79678
1982	18.10538
1983	17.8681
1984	17.86511
1985	18.12279
1986	18.0246
1987	17.85184
1988	17.36402
1989	16.61416
1990	15.72451
1991	15.62528
1992	15.71086
1993	15.73885
1994	15.75396
1995	16.17137
1996	16.58288
1997	17.07574
1998	17.18786
1999	17.12581
2000	17.62006
2001	18.62109
2002	18.47375
2003	18.35884
2004	18.22422
2005	17.81257
2006	17.30809
2007	16.57691
2008	16.13149
2009	16.03024
2010	15.73194
2011	15.90186
2012	15.72679
2013	15.97076
2014	15.53039
2015	15.08599
2016	14.67212
2017	14.74875
2018	14.75977