Year	Number
1970	28.86952
1971	29.14729
1972	27.37778
1973	28.53388
1974	29.00278
1975	29.29086
1976	29.01038
1977	28.37115
1978	27.97091
1979	27.98316
1980	27.24542
1981	26.59745
1982	25.68556
1983	25.38248
1984	25.03049
1985	25.24791
1986	24.86643
1987	24.35071
1988	23.62223
1989	22.78694
1990	22.31881
1991	21.93084
1992	21.98751
1993	22.07777
1994	22.37416
1995	22.85133
1996	23.29628
1997	23.73693
1998	24.15677
2001	22.20572
2006	18.25192
2007	17.68203
2008	17.55452
2009	17.21563
2010	16.84376
2011	16.78841
2012	16.84872
2013	16.85343
2014	16.23079
2015	16.29441
2016	16.54936
2017	16.59383
2018	16.42675