Year	Number
1970	21.23293
1971	21.56804
1972	21.34212
1973	21.31949
1974	21.7293
1975	22.09177
1976	22.17229
1977	22.42722
1978	22.71139
1979	22.60605
1980	22.2549
1981	22.36962
1982	22.33805
1983	21.77677
1984	21.26307
1985	21.23338
1986	20.51233
1987	19.76473
1988	19.08297
1989	18.78712
1990	18.21575
1991	18.04889
1992	18.14292
1993	18.10304
1994	18.44518
1995	18.77559
1996	19.28461
1997	19.77132
1998	20.30442
1999	20.80826
2000	20.58019
2001	20.37022
2002	20.30776
2003	20.04689
2004	20.44044
2005	20.20305
2006	20.09697
2007	19.49442
2008	19.77398
2009	19.66233
2010	19.65396
2011	19.50816
2012	19.40945
2013	19.32296
2014	19.39066
2015	19.17145
2016	19.04124
2017	19.01331
2018	19.02135