Year	Number
1971	19.46244
1972	19.94419
1973	20.21286
1974	19.7627
1975	20.26091
1976	18.86182
1977	18.34858
1978	16.52822
1979	14.90954
1980	13.97742
1981	14.82405
1982	14.16327
1983	13.87759
1984	12.83774
1985	13.22431
1986	12.88396
1987	13.53136
1988	12.15622
1989	12.08467
1990	12.30047
1991	11.35091
1992	10.61005
1993	9.97743
1994	9.19661
1995	8.90654
1996	9.1458
1998	10.54974
1999	13.12156
2001	12.59121
2002	12.35435
2003	11.69476
2005	11.11321
2006	10.68339
2007	11.11044
2008	12.50368
2009	11.22017
2010	12.03187
2011	11.27954
2012	9.59909
2013	10.0029
2014	11.20958
2015	11.62677
2016	11.6276
2017	11.77467
2018	12.1676