Year	Number
1970	15.55626
1971	15.96661
1972	16.30044
1973	16.29777
1974	16.19179
1975	16.44747
1976	16.93944
1977	17.3667
1978	17.62264
1979	17.38727
1980	17.19948
1981	17.07139
1982	16.90221
1983	16.94118
1984	17.07512
1985	17.39045
1986	17.31019
1987	17.24189
1988	17.08313
1989	16.88378
1990	16.71834
1991	16.8593
1992	17.16141
1993	17.46214
1994	17.67969
1995	18.03236
1996	18.24275
1997	18.55531
1998	18.71392
1999	18.72484
2000	19.11449
2001	19.72091
2002	19.69282
2003	19.53451
2004	19.61231
2005	19.25865
2006	18.92769
2007	18.50468
2008	18.06601
2009	17.82789
2010	17.81794
2011	17.95522
2012	18.0635
2013	18.80913
2014	18.68452
2015	18.43519
2016	18.01772
2017	17.81021
2018	17.83669