Year	Number
1970	31.74208
1971	31.78174
1972	31.11269
1973	31.4564
1974	31.49964
1975	31.25961
1976	30.93175
1977	30.33124
1978	30.01228
1979	29.94389
1980	29.36879
1981	29.21986
1982	29.09259
1983	28.35394
1984	28.57261
1985	28.8998
1986	28.77806
1987	28.95149
1988	28.65247
1989	28.65115
1990	28.79617
1991	28.75834
1992	28.517
1993	28.1087
1994	28.25324
1995	28.18039
1996	27.97105
1997	27.92818
1998	27.82139
1999	27.9269
2000	27.86096
2001	27.85119
2002	27.36357
2003	26.78102
2004	26.54158
2005	25.91793
2006	25.55919
2007	24.92175
2008	24.59782
2009	23.9517
2010	23.71606
2011	23.48743
2012	22.83637
2013	22.6816
2014	22.85168
2015	22.66048
2016	22.40378
2017	22.2065
2018	22.18372