Year	Number
1970	26.14272
1971	25.53549
1972	25.80108
1973	25.9969
1974	26.22289
1975	27.40714
1976	27.41585
1977	27.49019
1978	27.13443
1979	27.85294
1980	27.11775
1981	26.65604
1982	26.98037
1983	26.87602
1984	26.06062
1985	25.68272
1986	25.80887
1987	24.90529
1988	24.31776
1989	23.96977
1990	23.07136
1991	22.19506
1992	22.52248
1993	22.33735
1994	21.57236
1995	21.25102
1996	20.74145
1997	20.28183
1998	22.07269
1999	20.49885
2000	20.60623
2001	19.54309
2002	20.40191
2003	18.71187
2004	19.96165
2005	19.51261
2006	20.25374
2007	19.84292
2008	19.82279
2009	19.36558
2010	18.8199
2011	18.16637
2012	17.65605
2013	20.89611
2014	20.92848
2015	20.68384
2016	20.61597
2017	20.52872
2018	20.46768