Year	Number
1970	20.73626
1971	19.46076
1972	19.61421
1973	20.62827
1974	21.29066
1975	23.50529
1976	25.29223
1977	26.25349
1978	27.6334
1979	27.47775
1980	21.04608
1981	20.21812
1982	19.58662
1983	19.21178
1984	19.15225
1985	19.06167
1986	18.97215
1987	18.84893
1988	19.41262
1989	19.01746
1990	20.08438
1991	19.82173
1992	19.10912
1993	18.80945
1994	18.24956
1995	18.29652
1996	18.05883
1999	15.9968
2000	16.25611
2001	16.07911
2002	16.09095
2004	15.76138
2005	15.63104
2006	15.77361
2007	15.47421
2008	15.38675
2009	14.65648
2010	15.33901
2011	14.68664
2013	15.51877
2014	13.58912
2015	13.32796
2016	14.50099
2017	13.5809