Year	Number
1970	17.18881
1971	17.24297
1972	17.34613
1973	17.65258
1974	17.90663
1975	18.17051
1976	18.1558
1977	18.37521
1978	18.18609
1979	18.47578
1980	19.26342
1981	20.00406
1982	19.38358
1983	19.97564
1984	19.98061
1985	20.87739
1986	19.68188
1987	19.00912
1988	18.19292
1989	17.67893
1990	17.53695
1991	17.64418
1992	17.59993
1993	17.45457
1994	17.37471
1995	18.07015
1996	18.67241
1997	19.18382
1998	19.57902
1999	19.9012
2000	20.36357
2001	20.87854
2002	21.2088
2003	21.38787
2004	22.88128
2005	22.32893
2006	21.53834
2007	21.37032
2008	20.61831
2009	20.65859
2010	21.16983
2011	18.7994
2012	21.15492
2013	19.41601
2014	18.33013
2015	18.23693
2016	19.05278
2017	18.48555
2018	18.86477