Year	Number
1971	21.24135
1972	19.9978
1973	19.27926
1974	18.61053
1975	17.78251
1976	17.58176
1977	17.21282
1978	17.10593
1979	17.54662
1980	18.85859
1981	18.54175
1982	18.4582
1983	19.19339
1984	18.47636
1985	18.05843
1986	17.97516
1987	18.20206
1988	17.98267
1989	18.02722
1990	18.38603
1991	17.77026
1992	16.45486
1993	16.33404
1994	15.79244
1995	14.55094
1996	14.55227
1997	14.07525
1998	13.17911
1999	13.44605
2000	13.77575
2001	13.4826
2002	13.59342
2003	13.28756
2004	12.84563
2005	12.06221
2006	10.14275
2007	9.6101
2008	9.11151
2009	8.56188
2010	8.37512
2011	8.60901
2012	8.60056
2013	8.60512
2014	8.82856
2015	8.86229
2016	8.85539
2017	8.8586
2018	8.88073