Year	Number
1971	11.31359
1972	10.25764
1973	9.93822
1974	9.88209
1975	10.96318
1976	11.69963
1977	12.80105
1978	13.43182
1979	13.39425
1980	13.27629
1981	13.21059
1982	13.29964
1983	12.99842
1984	13.04272
1985	12.55306
1986	12.32914
1987	12.5699
1988	13.04761
1989	13.50254
1990	14.06524
1991	13.84433
1992	12.43003
1993	13.00386
1994	13.07303
1995	12.47614
1996	14.86626
1997	15.09937
1998	14.76194
1999	15.6737
2000	16.77803
2001	19.06907
2002	20.30484
2003	21.7398
2004	22.75336
2005	23.9429
2006	26.47082
2007	30.34938
2008	33.05663
2009	34.34736
2010	32.1359
2011	29.30955
2012	24.86631
2013	22.43357
2014	20.51349
2015	19.50356
2016	19.8604
2017	19.97092