Year	Number
1970	35.19
1971	35.72795
1972	33.78963
1973	33.75094
1974	33.48115
1975	32.74947
1976	32.23547
1977	31.36553
1978	31.7689
1979	32.66115
1980	31.06044
1981	30.6317
1982	29.93698
1983	28.02713
1984	28.15163
1985	27.89347
1986	27.38906
1987	27.82019
1988	27.36549
1989	27.21898
1990	27.3834
1991	27.34902
1992	27.19345
1993	26.52727
1994	26.79247
1995	26.64574
1996	26.25716
1997	25.87628
1998	25.10067
1999	24.00699
2000	23.79471
2001	23.56955
2002	23.35398
2003	22.87747
2004	22.35183
2005	21.8851
2006	21.08771
2007	20.09265
2008	20.51481
2009	19.8795
2010	19.36811
2011	19.60888
2012	19.33275
2013	18.73138
2014	18.60026
2015	18.62125
2016	18.51757
2017	18.22539
2018	18.92805