Year	Number
1970	22.16151
1971	22.1397
1972	21.67292
1973	21.56475
1974	21.2835
1975	21.01116
1976	20.82845
1977	20.32666
1978	20.04613
1979	19.77657
1980	19.53473
1981	19.41071
1982	19.25425
1983	19.12566
1984	18.93256
1985	18.55268
1986	17.96307
1987	17.8651
1988	17.58157
1989	17.44304
1990	17.04056
1991	16.48235
1992	16.27283
1993	15.93587
1994	15.35743
1995	15.63573
1996	15.54025
1997	15.3784
1998	15.47417
1999	15.23755
2000	14.6687
2001	14.37892
2002	14.02949
2003	13.95161
2004	13.86827
2005	13.68196
2006	13.56853
2007	13.3586
2008	13.27048
2009	13.16155
2010	12.94273
2011	12.76611
2012	12.90934
2013	13.14922
2014	13.32559
2015	13.29565
2016	13.09754
2017	13.20755
2018	13.203