Year	Number
1970	16.3724
1971	16.55593
1972	16.54953
1973	16.73056
1974	16.60498
1975	16.43934
1976	16.26046
1977	16.11026
1978	16.07317
1979	15.89509
1980	15.90956
1981	15.80219
1982	15.74799
1983	15.64674
1984	15.59675
1985	15.45381
1986	15.47404
1987	15.38202
1988	15.26138
1989	15.13456
1990	14.8202
1991	14.59764
1992	14.39093
1993	14.24198
1994	14.25802
1995	14.26653
1996	14.23544
1997	14.1977
1998	14.4148
1999	14.38388
2000	14.15634
2001	14.17686
2002	14.10732
2003	14.09832
2004	13.96108
2005	13.82927
2006	13.79355
2007	13.66902
2008	13.63471
2009	13.52443
2010	13.50957
2011	13.56167
2012	13.61118
2013	14.01954
2014	13.86628
2015	13.74787
2016	13.62644
2017	13.63641
2018	13.67666