Year	Number
1974	28.65497
1975	25.0999
1976	26.21564
1977	27.13397
1978	26.78614
1979	28.43682
1980	29.85501
1981	28.01972
1982	26.33308
1983	26.80853
1984	26.33372
1985	26.36408
1986	26.90835
1987	26.92974
1988	27.75315
1989	27.66304
1990	26.05641
1991	26.31043
1992	28.73313
1993	29.79399
1994	30.53278
1995	30.52224
1996	30.98329
1997	30.00607
1998	28.48124
1999	27.44985
2000	26.66438
2001	26.20314
2006	22.88608
2007	22.42936
2008	22.38452
2009	22.71068
2010	24.03674
2011	23.15033
2012	23.01143
2013	22.26595
2014	21.03872
2015	19.93141
2016	19.07577
2017	18.06286
2018	17.37685