Year	Number
1970	23.98021
1971	24.00578
1972	24.5657
1973	23.86591
1974	24.45502
1975	24.47371
1976	23.74899
1977	23.97768
1978	23.54418
1979	22.7311
1980	21.8931
1981	21.28271
1982	21.08596
1983	19.3092
1984	18.9879
1985	18.65407
1986	18.65567
1987	18.45763
1988	18.20692
1989	18.64578
1990	19.477
1991	18.7751
1992	19.02823
1993	19.32534
1994	19.99445
1995	20.02544
1996	19.58551
1997	19.46269
1998	19.7505
1999	20.02125
2000	19.85023
2001	19.72219
2002	19.78575
2003	19.5045
2004	19.39951
2005	18.80951
2006	18.16291
2007	17.45178
2008	16.95901
2009	16.46526
2010	16.30016
2011	16.16198
2012	16.06211
2013	15.62008
2014	15.70994
2015	15.878
2016	16.07018
2017	16.20945
2018	16.17787