Year	Number
1971	16.50738
1972	16.53313
1973	16.64682
1974	16.5017
1975	16.30754
1976	16.15421
1977	16.04494
1978	15.99039
1979	15.78966
1980	15.76855
1981	15.67894
1982	15.59639
1983	15.48448
1984	15.40815
1985	15.2495
1986	15.24817
1987	15.1343
1988	14.94311
1989	14.78764
1990	14.4746
1991	14.23519
1992	14.02775
1993	13.8469
1994	13.85746
1995	13.85754
1996	13.83484
1997	13.76029
1998	13.9215
1999	13.86463
2000	13.60712
2001	13.58426
2002	13.49115
2003	13.44495
2004	13.24807
2005	13.12839
2006	13.06176
2007	12.90337
2008	12.84772
2009	12.72533
2010	12.65978
2011	12.68246
2012	12.70481
2013	12.86842
2014	12.87004
2015	12.76727
2016	12.62698
2017	12.60643
2018	12.63098