Year	Number
1970	21.67139
1972	19.21478
1973	20.8203
1974	26.83813
1975	28.74452
1976	30.99085
1977	32.87237
1978	35.51412
1979	38.37667
1980	38.40719
1981	47.64196
1982	45.71431
1983	42.65393
1984	37.60126
1985	35.34697
1986	34.78435
1987	35.36696
1988	36.86349
1989	39.505
1990	39.1592
1991	38.7177
1992	38.93249
1993	38.84513
1994	39.13978
1995	39.48273
1996	38.57333
1997	37.83736
1999	39.04814
2000	38.03822
2001	37.01662
2002	39.86818
2003	35.65923
2004	35.82852
2005	39.71153
2007	40.01754
2008	37.81655
2009	33.30905
2010	31.91867
2011	29.61413
2012	27.5331
2013	25.63485
2014	23.93286
2015	23.09923
2016	22.42122
2017	20.90655
2019	19.74396