Year	Number
1970	31.92997
1971	31.98148
1972	31.22654
1973	31.90125
1974	31.92899
1975	31.9804
1976	31.79107
1977	31.86107
1978	31.59895
1979	31.56016
1980	31.23361
1981	30.85248
1982	30.35005
1983	29.81355
1984	29.56422
1985	29.49988
1986	29.23917
1987	29.0885
1988	28.83525
1989	28.2407
1990	28.22314
1991	28.14501
1992	28.1332
1993	28.13612
1994	28.25379
1995	28.24068
1996	28.24061
1997	28.29983
1998	28.39263
1999	28.39243
2000	28.05227
2001	27.5649
2002	27.19843
2003	26.27038
2004	26.47482
2005	26.24716
2006	26.10501
2007	25.93727
2008	25.78398
2009	25.37601
2010	24.97538
2011	24.63037
2012	24.70677
2013	23.81455
2014	23.45058
2015	23.27888
2016	23.77868
2017	23.64407
2018	23.67467