Year	Number
1971	35.31768
1972	37.66647
1974	38.50216
1975	37.28275
1976	38.16457
1977	39.02542
1978	38.44493
1980	33.4754
1981	33.10736
1982	32.42094
1983	31.74625
1984	31.07542
1985	29.69229
1986	29.5721
1987	28.62586
1988	28.10466
1989	27.63916
1990	27.02559
1991	26.04556
1992	25.20177
1993	25.0045
1994	24.50899
1995	23.86888
1996	23.62039
1997	22.14293
1998	22.83806
1999	22.3939
2000	22.35769
2001	21.90282
2002	21.58641
2003	22.26763
2004	28.30953
2005	28.87584
2006	28.09159
2007	28.39226
2008	27.08045
2009	25.85208
2010	25.37274
2011	25.20885
2012	25.25566
2013	25.28014
2014	25.17977
2015	24.93416
2016	24.9683
2017	25.02113