Year	Number
1971	56.87028
1972	55.97217
1973	54.65857
1974	53.07046
1975	52.29875
1976	51.78286
1977	50.24867
1978	48.80145
1979	48.63001
1980	48.09201
1981	47.52068
1982	45.51968
1983	43.8722
1984	41.66962
1985	39.93376
1986	38.30699
1987	37.87841
1988	36.6655
1989	36.36887
1990	36.2812
1991	35.5886
1992	34.43027
1993	32.81966
1994	31.25408
1995	29.70537
1996	28.39382
1997	30.91375
1998	30.90497
1999	32.22547
2000	32.12073
2001	32.024
2002	31.25796
2003	30.09795
2004	29.02175
2005	27.93159
2006	26.62451
2007	25.59446
2008	24.06701
2009	22.38299
2010	20.91797
2011	19.04941
2012	17.87581
2013	16.85167
2014	16.50455
2015	16.55424
2016	16.30703
2017	16.28669