Year	Number
1970	31.48826
1971	31.5833
1972	30.92861
1973	31.60625
1974	31.71275
1975	31.83181
1976	31.71242
1977	31.82907
1978	31.61023
1979	31.58993
1980	31.32151
1981	30.98692
1982	30.47396
1983	29.93277
1984	29.6687
1985	29.56153
1986	29.2741
1987	29.13315
1988	28.92684
1989	28.37272
1990	28.35973
1991	28.31649
1992	28.28284
1993	28.29471
1994	28.48032
1995	28.53621
1996	28.58813
1997	28.80701
1998	28.93644
1999	28.93467
2000	28.74273
2001	28.38908
2002	28.29544
2003	27.54779
2004	27.74522
2005	27.61426
2006	27.52692
2007	27.43755
2008	27.37657
2009	26.97649
2010	26.57106
2011	26.26128
2012	26.31432
2013	25.46278
2014	25.14388
2015	24.93134
2016	25.36889
2017	25.27992
2018	25.328