Year	Number
1971	16.18944
1972	16.21691
1973	16.33512
1974	16.19998
1975	16.01501
1976	15.86239
1977	15.76731
1978	15.6851
1979	15.48275
1980	15.49574
1981	15.41458
1982	15.34158
1983	15.21257
1984	15.14142
1985	14.99105
1986	14.98317
1987	14.88008
1988	14.69046
1989	14.5452
1990	14.244
1991	14.01079
1992	13.81534
1993	13.66553
1994	13.68019
1995	13.71283
1996	13.68467
1997	13.64927
1998	13.80138
1999	13.75809
2000	13.50455
2001	13.44587
2002	13.29174
2003	13.22752
2004	13.07111
2005	12.99244
2006	12.90703
2007	12.73566
2008	12.67935
2009	12.58006
2010	12.5144
2011	12.61838
2012	12.63678
2013	12.75882
2014	12.7433
2015	12.66088
2016	12.54451
2017	12.53682
2018	12.57776