Year	Number
1971	18.39777
1972	15.21449
1973	15.50505
1974	15.73813
1975	19.16026
1976	21.02069
1977	20.99685
1978	19.28766
1979	16.40383
1980	13.17103
1981	11.40951
1982	10.89052
1983	12.92422
1984	16.35175
1985	19.00884
1986	20.4629
1987	20.71573
1988	19.82047
1989	20.98091
1990	21.40248
1991	21.75639
1992	23.08405
1993	24.01266
1994	26.9119
1995	26.63655
1997	28.50224
1998	24.99294
1999	24.35996
2001	23.67985
2002	23.53677
2003	25.21334
2004	25.03251
2005	25.65717
2006	27.6334
2007	27.46772
2008	25.64265
2009	29.09617
2010	33.47657
2011	34.23549
2012	36.63395
2013	38.08324
2014	38.42064
2015	40.71702
2017	47.43572